We have very hard water here in Arizona so this is something I have to deal with from shower heads to fish tanks it's a pain. You can use a water softener or a filtered water that has a lower hardness. Depending on the fish this might not be a good idea as it will have different parameters. If that' s not possible you can clean your tank hood periodically with vinegar. It can cut through. There are chemicals that cut through it but I wouldn't use them on a fish tank just in case. Unfortunately if you have hard water in the tank you aren't going to prevent this without some weekly vinegar cleaning or chemical. Honestly, what I did, was stop putting hoods on my tanks. Anyway...weekly cleaning and vinegar are my advice.