Some reptile tanks are not made to hold water. If it housed a reptile but is made to hold water, then sure. If its not made to hold water, don't put water in it.
You mean the plastic trim part, or a cover/hood for the tank? If it doesn't have plastic trim and its a glass tank, its probably not made to hold water. If you mean a hood, you can buy one or get glass cut at a hardware store.
I also read somewhere that amonia from the reptiles urine can seep out from the caulk/sealent. No matter how well you scrub them. I wouldnt take a chance IMO.