So I caught an awesome sale the other day and couldn't resist purchasing. 1 dollar a gallon and 25% off of any aqueon HOB filter so I bought a 29g tank and a 30g filter didn't go oversized due to only putting one fish in and that's minimal when it comes to ammonia and bad bacteria. So after a week of cycling and filtering we put my son's beta fish in there. Day 3 he seems to be eating very little. From what I can tell his stomach isn't caved in at all and he seems brighter and more active. The question I'm having is, is there such a thing of having to large of a tank for one fish? Reason I really bought the tank is to soon put Rosie's in for when I finish my 90g tank which will hold maybe a few piranhas. Thanks everyone who helps out.