Everyone says you need RO/DI water to start a saltwater fish tank. I did not use RO/DI water for my first tank but everything has turned out okay so far. RO/DI water is one thing you have to have for certain types of tanks. Without a doubt, you need this water for a reef tank. However, depending on how you set up a FOWLR tank, you do not necessarily need RO/DI water. Some people have actually called my water a cesspool, despite the fact my levels were great. If you are thinking of setting up a tank using tap water, be prepared that you will not be able to have most corals and will have to treat bacteria and algae outbreaks. These are some corals, fish, and inverts that you can get with tap water.
Inverts:
Most all inverts will be fine in a tank using tap water, they are just more succeptible to disease.
Fish:
In my experience, fish are a lot like inverts using this water. Almost all are able to be in tap water but are more likely to get disease.
There are only a few fish that can't be kept in tap water and those fish are nearly impossible to keep.
Coral:
This is what really needs RO/DI water. Only zoanthids and palys are able to be kept successfully in tap. Everything else will porbably end up dead. Although, clams are sometimes able to be kept in tap water. They will just be stressed and possibly could die. Be careful with these though, if you have a different tank with RO/DI water, put them in there right away.
Inverts:
Most all inverts will be fine in a tank using tap water, they are just more succeptible to disease.
Fish:
In my experience, fish are a lot like inverts using this water. Almost all are able to be in tap water but are more likely to get disease.
There are only a few fish that can't be kept in tap water and those fish are nearly impossible to keep.
Coral:
This is what really needs RO/DI water. Only zoanthids and palys are able to be kept successfully in tap. Everything else will porbably end up dead. Although, clams are sometimes able to be kept in tap water. They will just be stressed and possibly could die. Be careful with these though, if you have a different tank with RO/DI water, put them in there right away.