Mag M is basically what Richard said, a mixture of Mag Chloride and Sulfate. Of course the biggest difference is that its 99% RO water. I know where I can get RO a bit cheaper than from Kent. I believe its Kent or one of the others that makes a dry Mag mixture. I used it before I started using Epsom. I always tried to use the dry stuff so I wasn't paying shipping, markup, and containers for water.

I sometimes add a little Iodine and Coral Accel to my tanks. That's about the only additives I buy and I probably won't be buying any more Coral Accel when I use up my 64 oz. bottle. If I could get iodine crystals I would make my own Lugol's. I have some Potassium Iodide.

Richard also has gotten me started using potassium permanganate from Kent. Its an oxidizer. It basically does something similar to ozone and acts as a manganese supplement as a side benefit. Its not something you should be using if you don't know what you are doing. Its pretty nasty stuff. The big advantage is that it helps break up detritus and lower TOC. My suspicion it that it can also increase the performance of a skimmer but I don't have any proof of that. I know that ozone does.

Gary