I kept 2 Dwarf Puffers in a 4ft 240L tank with all my angels and discus LOL.
Was awful! Neighbour turned up and said "here, you keep fish... have free ones!" adn shoved one of those really thin sandwich badswith water but no air, a double knot and two very cold and very unhappy puffers!!
I Couldnt put fish from my smaller tanks in with bigger fish in the bigger tanks so after i found the little devil puffers hanging off my discus fins (adult huge discus big enough to swallow 'the peas' were all hiding in the corner). Logical answer.... move all the discus and angels into my other tank and hope they all got on together (they did!) and left 'the peas' in the 4ft.
They lived in there for a month on their own LOL i got seriously bored of them
nasty aggressive little fish... id rather cuddle my stingrays, far friendlier!
But in all honesty, the only reason i actually moved them was because in a big tank, i seriously couldnt keep weight on them! I turned flow down, they ate from a worm feeder and got loads off food... but with too much space, they burn off too much energy and fat.. oh and they had been wormed btw...
Rehomed them to someone with a 30L tank. The pair of them did everything together and were just fine.
I think one puffer in a 25L tank with plenty to do is going to be more than fine!!
The trick will be finding a good enough small filter to cope with a fish that just isnt just really messy but its food is really high protein and manky! Hang on Back filters are great because you can put whatever you want in the filter to help