I do aquaponics so you can develop a ratio of fish to plants. If you are going to be dedicated to water changes you can over load the tank. A common ratio to a normal tank is 5 gal per fish. Your probably way over that but guppies are small. If you drop 2-3 gal per week you could probably load up about 15 fish. You'll need to get a water test kit, (strips or liquid tester), to monitor ammonia. Snails don't add much if anything at all. Though if they're pond snails,(little brown circular things), I'd get them out. They multiply like crazy and eat up the fish tank plants.
The water from the fish will give your plants something but not nearly enough as a stand alone feed for your plants.
How many plants you growing?
Here's a link for aquaponics. Just to be straight This probably nothing you would ever think of doing but there interesting to watch.
The 1st guy is a riot. Be careful, if you get the bug it can be a rabbit hole that can involve your whole family.
The 2nd is great for understanding nutes.