Marineland c530 is rated for tanks up to 150 gallons from what i have read and it would work if you only had fish . But with red eared sliders in the tank you are way under-filtered id recommend you buy another one and double your filtration not to mention the extra bio filtration will put your levels in check you can buy all the chemicals you want but the ammonia produced by a turtle is too strong and you will recycle your tank with chemicals forever trying to keep the levels down . More biofiltration is the best way to go and im sure much cheaper in the long run.
