Ok, I have been dealing with red slime for the better part of three months. It has me so stumped and fed up that I just want to sell the whole system. I have tried everything I know. First, let me describe my system. It is a 120 gallon acrylic mixed reef tank. I have about a 40 gallon refugium. It is the type that has the 4 socks, then the refugium, then water pools to the area where I have a large skimmer and Mag 9 pump. In the tank I have 2 Koralia 4's on each side of the tank (one on each side wall in the middle and they are pointed 45 degrees in and up), a Koralia 1 in the upper middle pointed towards the front, and maxi-jet with the rotating mod kit on a side wall towards the front of the tank. I tested my tank and it was: pH 8.2 Nitrate 0 Nitrite 0 Ammonia 0 Phosphate .50 Magnesium 1260 Calcium 440 dKH 12 Salinity 1.026 The only additives that I add are Kalkwasser, Magnesium, Iron, and Strontium. I have quit adding the Magnesium because I changed from Instant Ocean salt to Reef Crystals, and my magnesium level has been maintained with water changes. I quit adding Iron to experiment if it had any effect on the growth of the red slime as it does help macro algae to grow. I have a bunch of drama so bare with me while I explain. I have a MH fixture with 2-250 watt MH and 4-65 watt actinics. I didn't realize how long I had had them so they were about 15 months old when I changed them. I had red algae at this time and I began trying to figure out the root of the problem. I purchased 10K lights because I got advise to use those for faster coral growth, since I frag alot. So, I went from either 15K or 20K (I forgot) to 10K, but I did use the layers of screen for 3 weeks, removing a layer every week. I also tested my RO/DI water and realized it was time to replace my filters and membrane. I replaced all of that with new ones and the fresh water has since tested with 0 phosphates. I have maintained red algae through all of this. My phosphates in my tank were at 2 before I got my RO/DI system stuff changed. Now it stays at .50, which I have read/heard is not that bad. In my refugium, I had a 2-inch DSB made of refugium mud. I had a bunch of chaeto and other macro algae in there. It got all covered in red algae, but was also growing like crazy. This is what I've done over the past two weeks: I shut off the flow of my tank and took everything out of my refugium. Macro and mud were ditched. I washed the socks as I normally do about once a week or two at most. I took EVERYTHING out of my tank except fish and sand. I thoroughly cleaned sand as I did a water change. The water in the tank was a brownish by the time I was done. This resulted in a 30 gallon water change, plus everything I sucked out of the refugium. My tank is not covered in red slime, but it does film on the walls of the tank and I have noticed it on rocks and a couple corals. I have also noticed it on the front of my tank in the sandbed. I have done water changes and also been using Ultra Life Red Slime Remover and Chemi-Cleam Red Slime Remover (not at the same time) to no avail. Oh, and I have noticed Rapid Tissue Loss (RTL) on several of my SPS corals to include: Pink Stylophora, Lime Green Porites, Digi, and Millepora. I replaced 2 of the 3 lights in my refugium that were 6500K and now are 10K, there is still a 50/50 in there that I need to change, but there is nothing but water flowing through there for now. I didn't want to buy more macro right now just for it to get covered in red slime again. I can't figure out what is going on or what I am doing wrong. My test kit readings are dern near perfect. I do water changes every 1 to 2 weeks since this red algae started. I have 4 PH's in the tank, plus my Mag 9 pump. I also use a turkey baster to blow off the rocks every couple days just to make sure the detris is suspended. I have 5 fish (2 Damsels, 2 Clownfish, and a Lionfish) and a boat load of corals. I don't think I'm over stocked though. I have never dealt with red slime before but I don't know what else to do to get rid of it. Any help would be greatly appreciated. Also, should I get more Chaeto now or not?