When metals corrode, the effect is pronounced on the metals but negligible on the water, although it does steal some oxygen from the water.
Soluble metals are dissolved into the water from metallic salts, not from hunks of metal. Presuming you'll keep tabs on the water's ph, any metal being absorbed would show up immediately.
That said, the fish most desirable for fish tanks are all candy-asses, so it would be safer to talk to fish experts.
