"bk" wrote:

obviously a leak.

You can patch with any number of cements
or expoxies which work when wet.

Hydrolic cement can work when wet, but is that safe for the fish?

It is spelled hydraulic cement, more commonly called concrete or mortar.

Hydraulic cement, a cement or mortar made of hydraulic lime, which will
harden under water.

Hydraulic lime, quicklime obtained from hydraulic limestone, and used
for cementing under water, etc.

Hydraulic limestone, a limestone which contains some clay, and which
yields a quicklime that will set, or form a firm, strong mass, under
water.

Concrete, a strong hard building material composed of sand and gravel
and hydraulic cement and water.