the ceiling of the fishroom should be sealed with both visquene (plastic sheeting) and greenboard with a good tape job. The taping doesn't have to look good but it does have to seal. This is both for moisture and smoke in the case of fire. The greenboard will provide a slight measure of moisture protection over standard sheetrock but the true sealant here is paint. Don't worry too much about the type, even a standard latex will do, just make sure the coats (notice plural) are even and thorough. You should not be able to see green from the board.
The ventilation can be provided with a standard bathroom fixture. The idea here is not to move a whole lot of air at once, just a steady replacement. Unless you get truly carried away this should suffice. The important idea here is to vent the moisture to the outside much similar in fashion to turning on the fan when you take a shower in the bathroom. Moisture is NOT good for a house. They need to breath,especially the basement; fishroom or not.
Visquene behind the sheetrock will void any warranties . It needs to breath from both sides.
Any latex will do but flat paint has no "Shell" so to speak. The higher the gloss the more protective it is against moisture. That is why they use semi- and high gloss in kitchens and baths.
The ventilation fan will be real helpful if it can remove heat as well as moisture. I have 110CFM bath fan in my tank room and it barely does the job in the summer.I also have my fan set up with a cooling stat mounted to the ceiling. It turns on automaticly when the temp get too hot. Spend the extra money on a real quiet bath fan or it will vibrate the whole house. My fan runs even now with temps in the 40s outside.
Make sure you have some fresh air intake to replace the air the fan is trying to remove.
Moisture is real bad for a house. Ther is such a thing as too air tight as well.
IF you can, put a sink and drain in your tank room. I have one of my overflows plumbed straight into my house. Real easy water changes.
A counter top is helpful.
Storage for testing supplies.
Leave room for your equipment to stretch out if you can.
Make sure you have access form the front of your tank as well as from the tank room. It makes aquascaping and maintance like nuking aptasia easier.
Tile floor is forgiving of spills.
Pick a nice paint color as you will be spending alot of time in there.