will zinc screws corrode/rust?

I am not sure how dangerous zinc is to fish (if at all), but like the other post said just smear the screw head in silicon. The remainder of the screw will be imbedded in the rock and wood so the exposure is very minimal.

The problem with zinc plated screws is that you usually scratch through the incredibly thin plating just screwing them in. Most run-of-the-mill no name hardware has a barely adequate plating (read “a few atoms thick).

Alternatively your DHS (distant hardware store?) should have stainless screws.

Gumby

I know that in my boats engine i have zink plugs that slowly eat away. The water attacks them firs to prevent corrosion in the engine. I would stay away from the zink coated ones and go with the S.S. ones.

IMO, no metals are safe in tanks. They will all corrode overtime.