I set aside an amount of $ for fish each month. And have several pairs of birds, parakeets and finches that put out chicks each month. Sell them at pet shop. Don't get much, but pays for seed and some extra for fish. Buy fish food in bulk on line, so other months I can buy favorite fish, but sometimes I can't because I have to buy equipment, like replace non functioning heater. Or upgrade to a better filter because of breakdown. (Always keep a couple spares--not the greatest but do the job.) Some months I don't buy anything, then there is a little extra for next month. Sometimes I overspend--less next month. Some of Birthday money goes toward fish.
Can be expensive hobby. I buy SAFE water conditioner. Dry, Concentrated Version of Prime. Same company, Seachem, same product lasts a long time. 6 month--1 year for my multiple tanks. $10 for 6 months instead of $7-$10 a month. Extra $10 a month can go somewhere else. Am trying to breed fancy Bettas now. Used to do it and make some extra money.