Prime will neutralize any chlorine, ammonia, and chloramines in your tap water. This is important because all three of those are toxic for fish and inverts. However, it's possible that there are many other things in your tap water that could be bad for your tank. Anything from heavy metals, which could be toxic for inverts to phosphate and silicates which could cause a lot of algae. While prime can make tap water safe it is definitely not a replacement for an RODI filter and in most cases people who just use tap water run into a lot of issues. Nuisance algae is probably the biggest one.
That said, there are some places where tap water is very pure and there are some people who use tap water with few problems. However, that is more the exception than the rule.
Unless you know what is in your tap water or you know of other reefers in your community who get good results just using tap water then I would strongly suggest that you get an RODI filter or purchase distilled or RODI water to use rather than just using tap water.
