You have some stocking issues if you have an oscar and 6 cichlids in a 55 gallon, but a pool filter will most likely not be good for your fish and have the necessary filtration, but I'm not sure.
It's a canister type.very small it couldn't keep the small pool I bought clean so it's not any thing too heavy duty. But I've done some reading on using them in tanks, just wondering if anyone here has any tips or success at all.
What kind of canister is it?
Agreed that you have stocking issues (that will likely end up with the fish killing each other if you don't change it), and if you don't have a filter on the tank now, you should be doing large daily changes to prevent your fish from dying from ammonia poisoning.