So I have a 5.5 gallon aquarium laying around, and I LOVE seahorses and would love a pair or two. I don't imagine it'd be too expensive to keep up, being a small aquarium and all, but is it? I'm scraping by right now. Also, how difficult is it? I find some sites saying they're difficult, others saying they're easy. Also, the only kind of sponge filter I am aware of is the kind hooked up to an airline, and this is always noisy in my experience. Is there another way to filtrate the tank? And what kind of substrate do they like?