Can those magnets effect the water?

They do not, the only time that they can is if the metal is cut or damaged which will make it rust (although that specifically will not cause any issues).

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Nope, there shouldn't be any issues

No, there should not be any issues at all when it comes to the magnets that you use in a tank for things such as you mentioned. These will have been made specifically to be safely used within this environment so that they don't rust or cause any issues in the water. I would definitely replace though if they ever get damaged as once they are damaged, you will find they may end up causing issues.

No, the magnets used for cleaning or holding plants together in a tank should not have any negative effects on the water. These magnets are specifically designed to be safe for aquatic environments and should not rust or alter the water parameters. However, it is important to replace them if they become damaged to prevent any potential issues.