I need some advice on apple snails because I'm planing to get some for my 55 gallon tank.Here are some questions.

How long can an apple snail live?

How many do you think I would need to keep a 55 gallon tank with goldfish clean?

How big do they get?

Those are all my questions.

If you're planning on using the Apple Snails as a vacuum cleaner for your messy goldfish then they probably won't live long. You need real food for them, like algae wafers etc.

How many goldfish do you keep in the 55 gallon? Do you have a filter that is double the filtration capacity? Do you keep up with 40% water changes every week? Goldfish are very messy.

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i am going to have 3 fancies and comet

Sounds good. Make sure to keep up with water changes and such get a stronger filter when you can. The goldfish will probably try picking on the snails but will give up when they can't break their hard shell.

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Do you have the fish already? It's not usually recommended to keep fancy goldfish and comets together as comets are larger and faster s might bully the others. I have seen it work but it's a risk.

As for Apple snails and goldfish, it's a bit tricky, sometimes it works but goldfish often pester the snails too much, especially as the get larger. It's also hard to feed the snails as the goldfish will tend to gobble everything before the snails can get it. In truth although they eat some algae they won't really do much to actually clean the tank but they are cool to have.