Just the rock for a filter. Maybee this is a problem. In a FO tank I have fount it to greatly help to have carbon filtration also. Carbon is supurb at removing chemicals. Even if you don't add any treatment stuff, there can still be some from the water. Are you using tap water? I do and havn't had any problems. I know I'm supposed to do the RO/DI water, but if it's not broken I don't fix it. I know the city adds flouride to our water supply, so what else do they add. The carbon takes it all out as far as I know.