New 40 gallon breeder set up--undergravel filter?


Hello! I currently have a 20H tank set up and it is getting overcrowded so I am transitioning to a 40 gallon breeder. My current tank has a Fluval 30 filter and an undergravel filter set up. I have a pleco, 2 Corys, a serpae tetra, 2 glofish tetra, a diamond tetra, and 6 harlequin rasboras.

I bought the Fluval 50 filter for the new tank, but the undergravel filter I purchased is too narrow.

Has anyone set up or found an undergravel filter that fits a 40 gallon breeder? Is one necessary?

Thanks in advance for any tips.


Fluval AquaClear 30?
Undergravel filter is not necessary and has an issue where you cannot really clean it without tearing the whole tank down. But you will need another filter on top of the 30 assuming its that hang on back one. Maybe a canister? I only rinse mine when the flow gets slow maybe once in 3-4 months


your stocking chose may have not been that wise, tetra need to be in minimum schooling of 6. to the serpae, glo and diamonds need more in numbers to feel safe and happy you cant just have 1 or two of a schooling fish, same with the cory they are also school and need some friends minimum 6. DO you know what type of pleco it is some will outgrow your 40gal.6 hal rasboras is a good number to strat with if you want maybe add some more.

In regards to undergravel filters they are now often seem as an old method of filtration so they arent as popular but perhaps search on amazon and other online retailers? ive never used a under gravel before so can reccomend any.
best of luck


I would put the Fluval 30 (just how it is not cleaned to keep most of the cycle for the fish you have.) AND the 50 both on the 40. Forget the under gravel filter its not necessary. Dont add any fish for a month or more so the 50 can get seeded good first.


What size is the one you have? If it only a little smaller you can still use it, especially since you'll be running another filter along with it. Think of it this way: Running an UGF under the entire substrate works fine, and not running one does too. So even partial coverage will not be a problem.

If you want a full 36 x 18 unit Lee's offers one.


Thanks for all of the helpful comments.

The UGF I bought is a 36 x 11, and my new tank is a 36 x 18. So if I used it with the other filters, it would only cover part of my base. The only tank I have had so far has a UGF so I wasn't sure if that was really necessary or not. (I knew nothing when I set up the 20 gallon). I appreciate all the suggestions.

I think I will skip a UGF filter and use just the fluval filters instead. The fluval 30 has done a really nice job on my 20 gallon and I was planning to move that to the other tank to seed it. thanks for the suggestion to put both on the new tank. Would both need to stay on the 40 gallon indefinitely to keep it properly filtered/clean enough, or just for the the first month or two until the Fluval 50 was seeded with the bacterial needed for the new tank?

As for the stocking, we did end up with some odd numbers. We started out with 6 serpae tetra and 4 glo fish tetra and danios when the 20 gallon tank was a birthday present for my daughter two years ago. There were some aggressive fish and lots of fighting/damaged fins and fish drama/death. The lone serpae and lone diamond tetra are a result of that. The four tetras that have remained hang out together peacefully so I might add some more tetras as well--but am always leery of adding an aggressive one into the mix. Right now we have a very peaceful tank mix with no fish aggression/nipping. I would like to add more Corys--when I got two, I didn't know much about them and particularly their need for a school, so that is part of why I wanted a bigger tank to give them a school. The rasboras I added recently and those are fun fish we have really enjoyed.

I am not sure what type of pleco it is--it is a very pale albino/light yellow/orange color. I don't see a picture of him I can attach right now. I only have a video handy and he/she is elusive/hiding right now. I think I recall the fish store saying it was a dwarf Bristlenose and will send a picture if I can get one.


I would leave both myself on the filters. Over filtration is always ok. The 50 filter may not be enough for the 40g.


Thanks. I have not opened the Fluval 50 filter yet--I could still exchange it for the Fluval 70 if that might work on its own?


Yeah it should. I would run both for a month or two though still.


Will do. Thanks!


My pleco reappeared—hopefully he/she won’t outgrow the 40 g. About 4 inches now.


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