Hey there! Yes, you could do a puffer in a tank this size. I'd say just 1 though. If you really want to do plants that require Co2, then go for it. I wouldn't do DIY, levels are just too risky in a small tank like this. Regulated Co2 is a different story though. Here's some general care for puffers:
I have 2 in my 5.5 and they get along just fine, with the occasional territorial dispute. I also keep them with a lot of RCS, but I don't expect to see lots of shrimplets. They do leave the adult and juvenile shrimp alone though. There's 1 Amano shrimp in there as well and she's 2 times the size of the puffs so she gets left alone of course
I wouldn't dare add any other fish to the tank, even in a 10 gallon, let alone a 5.5 The only fish that they always get along with and many have reported successful with are oto cats. They stay small and eat lots of algae. For some reason the puffs leave them alone. Maybe because of their dull colors and they don't move a lot?
DP's really do appreciate a planted tank. My 5.5 is very heavily planted, and they are constantly exploring and checking things out. One way to know if the puffs are bored is if they are "glass surfing". This is when they swim up and down or against the glass. My puffs do this whenever I enter the room because they are begging for food of course lol. They are highly inquisitive fish, very intelligent and very interactive as well. They peck at my hand during maintenance, beg for food, and know the difference between me and my siblings, mom and dad, probably because they know that I give them food, which brings me to the next subject of discussion
Dwarf puffers are listed as an intermediate fish for a reason. For me, that main reason is feeding. Most will need live food at some point. Some will accept frozen food that is waved in front of them for a while. Never, ever ever ever have I seen a puff accept flake or pellet food and I don't think I ever will. You can keep all kinds of live food cultures for them, like black worms (what I have right now), pest snails that grow and reproduce quickly, like pond, ramshorn and MTS snails, shrimp (RCS shrimplets) this may not be the best option because I would rather have shrimp in the tank to enjoy, and they cost money, which means you can make money off your live food
My blackworm culture is really simple. Just a small plastic container (just a small tupperware tub) and some plastic mesh for them to hold onto. Got a big sheet of the mesh at hobby lobby for 99 cents. Add your worms and just change the water every couple days. Don’t feed or nothing. Works perfectly for me.
Overall, they’re great fish. Pretty easy to keep IMO. Only the feeding is a problem. If you’re willing to go the extra mile for that, go ahead and try them!
