The market for fish is very inconsistent. It always seems when you have fish to sell, the market is swamped, when there is a market you have fry the size of a speck. Eight weeks later when you are ready to sell, it's swamped again.
I spend around 3 hours on the weekend with water changes, tank cleaning & maintainance, along with anywhere from 10 to 60+ minutes daily on feeding & other fishroom related activities. My electric bill is around $50 more per month, water is maybe $10. I buy food in bulk, probably sets me back $15 monthly. Add in another $20 monthly for dechlor & other supplies. I figure it's around $100 monthly to run 14 tanks, 380 gallons total. This doesn't count in setup costs, I didn't figure that, as your setup is paid for.
It's mostly an angel setup, I get $1.50 each for juvie angels if the market is good, a buck for 3 if I can get rid of them at all if the market is swamped. Once in a while I make over $200 a month, some months I bring in ziltch. If I break even, I'm lucky. This just covers fishroom costs, not the hours I work. Reading the market for fish is nearly impossible.
You could give it a try, I have. I've found it mostly gives me a reason to hang out at auctions & swaps, looks like I'm working when I'm really enjoying myself, pretty much goofing off. If I really pushed it, I could probably get a more consistent profit going, but I'm enjoying myself as it is.
Tolak