One little puffer is the maximum you could put in a 4 gallon. They are very territorial.

I vote for the HOB filter since it provides extra water volume and aeration.

 

I've done my researching on google and puffer sites as well. It appears 3gal is min but I have a ~4gal. I say 4 gal because when looking at it beside a 5.5gal it just somehow eyeball looks like a 4 gal a couple cm smaller all over.

I am aware the tank can only house one. I should have said in my first post to clear up some confusion is that I plan on starting with one DP in a small tank then if I choose to get more later and I have a free 10gal to work with them I'd get more.

I have read before that if you go to small a tank then yes the DP will survive but also get very aggressive. Also the territorial thing I've also read it's not always that bad as with the aggression. It seems that yes overall it will happen but I've read of cases where the DP was tame probably the personality of that DP.

 

Since you only have a DP in a 4gal tank than either one will do the job but if you don't like the noise from the air pump than go with the HOB. You will lost some tank space if you go with the sponge filter too so I would go with the HOB.

 

I was also thinking Hang in Tank (HIT) filters as well but I'm not sure if there are any HIT's with the level of media flexiblity of the AC20. I've fallen in love with the AC20's because of flexiblity and many DIY options for the filtering. Yah HIT loses some space.

I'm also considering a design change and wiring a high power LED into the tank and have a foam floating top with about 3-8 holes in the foam for above water plants and dual purpose that small tank as a hydro/aqua ponic cloning tank on top. One things for sure I'll be dropping some live plants and if I go with the cloning top the roots will hang down and clean more of the water nitrites out while giving the DP LOS breakup/floating hiding places.

 

I would go with the HOB or something els external. Since it gives you more room to work in the tank.

 

Ok, I'm going with HOB. I'm thinking center position filtering would work out better should I decide to put the cloning top on I could cut out some foam so the filter flow can go back into the tank. Now thta got me thinking about a mini canister filter.

 

i've got two DPs in a 6g and was dissapointed I'd chosen an in-tank filter (to account for noise - it's in my bedroom.)

it's such a small size, the filter really eats up room.

aggression wise, they're quite ok. been so for over a year. believe i've gotten lucky though.

 

From all I've read it may depend on how the puffers personality is. Some just will fight and fight even if you put them in a 10-15gal they'll go after the other DP's from what I've read. Yah perhaps your DP's are mellow or got to be buddies in the tank or both are getting their fair share of food which probably is why they arn't having any issues. I've read before on some puffer/fish forums that sometimes the aggression can be sparked by not having enough food and then anything else moving in the tank (shrimp/etc) tends to get picked on or eaten.

 

You will want it very heavily planted to break up line of sight.
And IMHO, go with hang-on, not -in. In's are a PITA to change/clean media and not as flexible as a good old AC. And lot of puffers stress when you mess with things *in* the tank...