How long has the tank been running? Do you clean your canisters regularly? If clogged they can go anaerobic and cause more nitrates and ammonia. Did your sump have all of the bio-material in it? It could be removing the sump and replacing it w/ a un colonized canister is causing an ammonia spike.
Also what's the PH? I know discus like softer water, if its very hard that might be an issue
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