I have had a tank for about 4 or 5 years. It has a wet/dry system with bioballs. I chnage filters, etc thru the years but was told when I bought it that you would never have to chamge or clean bioballs. I heard somewheret that you should clean the bioballs periodically. I do not want to clear off any helpful bacteria. SHould I clean them or not? If so, how ?