I have 4 pea puffers in my 20 high. I use the tank as a plant grow-out tank (trimmings from my 55 go in there until they are big enough to give/trade to local people). They are really fun. I feed mine snails from the 55, and when they see me come up the stairs, they will all swim to the glass to "beg."
Great job with the tank. I think the layout/plants you have make it look bigger than just a 5.