Yes it would be okay to do a water change. Try to make the water the same temp. as the tank and you should be okay. They will die if they are forced to shed to often. The problem with your aiptasia moving about is that everytime it moves it leaves a little bit of itself behind and that grows into another Aiptasia.
Water making that much of a change can and will stress your tank animals.
Clowns are weird fish. They will host in almost any coral or duster that they find handy. I have seen them in dusters, GSP's, hammer corals and frogspawn corals. I'm sure the list goes on.
Remember one thing and that is to go slow. The only thing that comes fast to a saltwater aquarium are mistakes.
Here is a picture of what I use to help reduce the temp. in my tank. Be aware that this causes lots of evaperation so you have to check your water level daily. Also remember that only the pure water evaperates and not the salt. So just add R/O water.
