I'm considering purchasing a used canister filer that was used for a saltwater application, can I now use it for my freshwater South American Cichlid tank ?
I would think if the filter is in good shape, & you clean it out good, & make sure the inside components are in good shape, or replace if the sponge has deteriorated ( for example ) the filter will work fine for your freshwater tank, as long as the flow is not too strong, or weak for the size of your tank...
We used to run a saltwater tank for our puffer fish Dotty the canister filter that was in there was donated and is now in my sisters freshwater tank and she has had no problems at all.
As others have said, it will need a thorough clean but it should be fine.