Seachem's product Prime can be used as a quick fix to remove nitrates and ammonia. However, it is not a long term fix as all it does is change the chemical structure. If used repeatedly without removal via waterchange I imagine it would reach a maximum and convert suddenly back into the toxins.
Just a bandaid till you can stitch the wound.
Also do what works for you, no need to go spend hundreds on dollars on new equipment if your current equipment has been working just fine. Just remember that you will need to think twice about future purchases and if your canisters can support them. A stingray was a bad choice. I think you are realizing this now, you just doubled the workload you are going to need to preform on the tank (ei: much more frequent water changes or you will lose critters). Now, Groupers/Lions/Triggers would also be bad choices (due to the waste they produce) so keep that in mind. A canister just doesn't house enough bacteria to provide for drastic increases in bioload. But you should be able to house smaller specimens just fine. You will just need to be more careful and attentive than the average hobbiest.
