New to this website, and to keeping fish, I have a 100gal, no live plants, I've learned about cycling the hard way and have done a lot of research on fish keeping lately. My Ammo is 0, Nitrite is 0, Nitrates are 20, I'm having a problem with two things, PH and, from reading a post in this website, Diatoms. After a 50% water change the PH is 6.8 to 7.0. Then in about a day or so, PH starts droping quickly. Third day or fourth is 6.0 or lower. How can I keep the PH at around 7.0? and what do I do about the diatoms? Any help would be appreciated. I got this aquarium for my wife for Christmas, I told her it was good for her stress, that didn't work out too good, she loves her fish and I have to do all I can to keep them all happy. Thanks,,,,....