Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55)

I’ve been using these Penn Plax Premium Gravel Filters across all of my shrimp tanks, and they’ve consistently proven themselves as a reliable choice. The construction feels sturdy, and the design is straightforward, making installation simple and hassle‑free. Once in place, they fit neatly under the gravel and provide steady filtration that keeps the water clear and healthy. What I appreciate most is the dependability and ease of use. They run smoothly without needing constant adjustments, pulling debris down into the filter bed and helping maintain stable water conditions. For shrimp tanks especially, this kind of consistent performance is important, and these filters deliver exactly that. The materials feel durable enough to last, and the design works well across different setups. Because they’re so reliable, I’ve made them my go‑to option for every shrimp tank I maintain. They blend into the background, but they play a critical role in keeping the environment clean and balanced. A well‑made, dependable gravel filter that’s easy to install, runs smoothly, and keeps shrimp tanks clear with minimal effort. I trust them enough to use in all my tanks, which says everything about their quality and reliability.

An under gravel is simply one of the best filters you can buy, dollar for dollar. This is because they are highly effective, simple, inexpensive, and last a long time. Here are some tips for how to use this filter correctly. 1. DO NOT OVER CLEAN YOUR GRAVEL. This is the most important thing. You should not clean your gravel much at all. Yes some brown stuff will accumulate. This brown stuff is not "fish poop". It is beneficial bacteria. If you remove this beneficial bacteria your filter will stop working and your fish may die. DO NOT OVER CLEAN YOUR GRAVEL. If you must clean your gravel, only clean 1/2 the tank at a time and give the cleaned side some time to regrow its bacteria before cleaning the other side. Otherwise only very lightly clean the top 1/2 inch of gravel. 2. You do not really need the carbon cartridges. If you want to support Penn-Plax, I guess you could keep buying them. 3. Use 1-3 inches of 1/8 inch aquarium gravel on top of the filter. Under gravel filters are so effective because they provide a lot of surface area for beneficial bacteria to grow on. The more gravel you have the more effective this filter will be. 4. Decide whether you want to use a power head or an air pump to move water through the filter. Power heads will greatly increase the effectiveness of the filter and provide cleaner water. If you plan on having a lot of fish in your tank, you should use power heads. The power heads should be rated at a gallons per hour that is 2-3x the gallons of the tank. If you use air pumps, they should be rated for the size of your aquarium. 5. Decide whether you want to join the under gravel plates or not. If you are using air pumps I would not join the plates. Simply put an air stone in each uplift tube for each plate. You do not need to buy multiple air pumps. Just buy a simple 'T' joint to split the output from the pump. If you are using power heads, I would join every two plates and use one power head per two plates. Some people are complaining that this filter killed their fish which is very unfortunate. The filter did not kill their fish. Incorrect use of the filter killed their fish. They probably cleaned out all the gravel and removed all the beneficial bacteria which caused the filter to stop working. Unless the filter has become so clogged that the water is no longer flowing through it, there is no need to clean the gravel or underneath the filter. If this does happen, I would suggest that the person is greatly overfeeding their fish! Some people are complaining that this filter "produces high nitrate." Producing nitrate is the point of all filters and shows that a filter is functioning properly! If your nitrate levels are too high for you to manage, you are putting too much food in the tank. A different filter is not going to fix this. There are only two ways to remove nitrate from an aquarium: change the water or grow plants. Really the plants are just incorporating the nitrate into their bodies, so you will have to remove some plants eventually too if you want to actually remove the nitrate. There is a reason many successful hobbyist have used under gravel filters for decades and I am very glad that Penn-Plax makes this product!

Filter seems to work fine and was easy to put together, but one of the stones has something wrong so it is not properly pumping air. Other than that it seems to work. Will update once the aquarium is established and has been in use for a while.

It works great just like it’s supposed to. I didn’t realize how they worked until I set this up. I like it better than having the other kind of filters.

Good product well constructed

I've had aquariums since the mid 80's, and at that time the UGF was king. I still think the UGF is far superior to todays HOB and canisters for many reasons and a pain for others. The reasons I like UGF over HOB or canisters: Nothing matches the filtering capacity of your entire aquariums bottom for mechanical filtering, and as the void under the gravel is where the good bacteria live, you'd be heard pressed to find any other type with the surface area of the UGF for bio-filtering too. The carbon filter at the top are cheep, easy to replace and if your so inclined, can be replaced for a power head or even opened up for replacing with new carbon (not intended to be used as such, but were all tinkerers here, right?) What I dislike about UGF: Over time the bottom can, theoretically, become backed-up with waste. If you overfeed or don't have plants which can consume some of this, it can back-up quicker. However, a gravel vac can remove most of this while doing water changes and in my experience, I've gone many years before I've needed to remove the UGF, usually I only remove it and clean when I get tired of the tank set up and wish to redo the tank.... and even then, I do it not because it's needed, but because I'm in there already. This is maybe not a dislike, but as it's not easy to do, I list it under dislikes. Over time the tubes usually discolor, become dirty or the like, so over time they will need to be pulled out and cleaned. This isn't hard to do, but it a bit more involved than cleaning a HOB, but is mostly for athstetics, unless the tubes got too dirty and cause a blockade after years of use. The final item is by far the biggest and only real dislike I have for them, and should serve as a warning for others, with a fix at the end. If you have a tank where the fish breed, the eggs can get pulled into the gravel and wind up at the bottom of the tank, under the gravel. The eggs hatch and now you've got fry swimming under the gravel, had this happen to me. As designed, there is no way for those fry to get out of the system, as the gravel is overhead (can't borrow through that), and the path to the tubes is blocked off even if they could somehow get through the carbon filter (they cant). So this was the reason I stopped using them many years ago. Was the only reason I stopped using them. Now, to fix this...... The plates each come with 2 spots to put tubes in, most likely you will not use or need all of those spots. Each plate is connected below (when knock outs are removed), so fry can swim between them, simply cut one of the unused riser tubes sections out and insert a use in it's place as an escape. I used some tubes I had laying around to do this, but I leave the top capped as to not create other issues. If I see fry swim up the tube (which they will), simply open it and let them out. While this is an issue, it's kind of a double edge issue.... bed they get trapped, but as they are trapped with nutrients below they are not food for others..... have had more than a dozen live down there before and go one to reproduce themselves. How many I lost before this fix... I don't know. On to the 3 star ratting. This is a reflection on Penn Plax build quality and changes. The "old school" design (brown plates vs blue) were made of a very ridged plastic, This was a bit brittle but posed no issues after they base was installed. These new blue plates are about 3 steps backward. Since they are not ridged, they sag from the weight of the gravel on them and even have give from just the riser tubes installed.... 2 issues the older design, which I still have, does not suffer from. Only time will tell if this leads to more than just crooked looking tubes after it's installed, but the fact the tubes are crooked for some might be enough to avoid/return this.... I almost did, but I have plants to cover it and added rock under it to help stiffen it up. The second star was removed as the riser tubes are not as long as they should be or once were. I have a 49 gallon bow front, on all my old UGF's the tubes rise high enough to bring the top of the heads just out of the water a bit, making it perfect to filter and reduce the gurgling noise. This new design leaves the tubes 2+ inches below the surface while fully extended and as such you get the bubbling/gurgling noise. I do love UGF's, but I do not know that I could recommend these over the other brands out there now-a-days. In the past, it was Penn Plax hands down, but with these 2 issues, I feel I will try another brand when I set up another tank if these are not fixed before that time. Hope this helps anyone looking for an UGF.

Well made under gravel filter. I used it in a 40 Gallon tank I'm just starting to set up. I used three of the four supplied bases and my tank is 36 inches long. Everything was there taped up. The plastic is perfectly adequate for the job. There is no need to go heavy duty on these parts. They will last decades as is which is more than can be said for the HOB filter variety. Just getting back into the hobby and I learned my lesson well when the HOB filters became the preferred profit makers for filter companies years ago. Once a very popular filter, actually it was considered the best at the time, it is now frowned upon and is no longer considered the best filter to use. This is not because the filter is not great at what it does, its just that the definition of "best" has been refined for modern times. "Best" is now defined by a product that requires regular purchase of filter media to get the "best" profits for companies producing them. They are also "best" at wasting resources and adding to landfills. They are also "best" at unreliability being at about five years or so. So if you want the "best" available today in terms of all those things, don't get this. This filter won't create waste for land fill, won't fail spectacularly, won't require you to keep the filter companies profits afloat, but it will filter your aquarium crystal clear for decades without issues or monthly costs. Just understand how they are meant to be maintained (leave them alone!) and you are good to go.

These UGF are great. Much better than all the other filters out there. I had to tear down 2 tanks as they were dirted tanks and all my plants started died after being in the tanks for 2 years. I switched out the dirt and sand for a layered substrate on top of the filter plate. 1/4 inch - 1/2 inch Volcanic rock, just enought to cover the plates, then Safe T Sorb on top of the Volcanic rock. Then 2-4mm gravel. The plates fit great in my 20 gallon. And I bought another 20 gallon UGF. I used 1 of the plates for my 7.5 gallon Cube tank and it fit perfectly! I'm saving the other plate so I can use it in my 15 gallon cube tank. All my tanks have lots of live plants in them. So I just add a iron with potassium, liquid. So just 2 pumps in my 20 gallon and a half pump in my 7.5 gallon cube tank. I was able to save a few plants with this stuff and the UGF! No other filters for me!

This was easy to assemble and has performed very well and the price is very competitive.

Works really good for my 2 footer tank

Super passgenau für meine zwei 25cm Becken. Funktioniert wie es soll. Günstig, aber gut.

Very good product

Muy buen servicio del producto, lastima que los tubos de ensamble venían los 4 estrellados, no rotos, lo bueno son funcionales, pero deben cuidar más el empaque porque se nota un mal manejo del paquete o los empacaron estrellados y eso deja mucho que desear de la seriedad del vendedor