After spending too much on dry supplements over the years, I recently switched over to a DIY two-part very similar to one Randy Holmes-Farley recommends in his article,
An Improved Two-Part, Do-It-Yourelf Calcium and Alkalinity System. I based mine off of recipe 1 (which uses sodium carbonate instead of sodium bicarbonate for the alkalinity part to bolster pH) and part 3A for the magnesium supplement. Since I have access to lab-grade chemicals, I used these to make my solutions. Also, in science we work in moles, not parts per million, so I modified the recipes given in the article by scaling them to easy molarities:
part 1: 1 Molar calcium chloride (CaCl2) + 0.037M potassium chlroide (KCl)
part 2: 2 M sodium carbonate (Na2CO3)
part 3A (5x concentration): 0.633 M magnesium sulfate (MgSO4) + 1.04M magnesium chloride (MgCl2)
I generally try to add 5ml per day of parts 1 and 2, so that means 1ml per day of part 3A. I usually forget for a few days, so I just add about 15, 15, and 3ml whenever I remember. I've had better results with the DIY 3 part, mostly because making it forced me to research reef chemistry and come to a better understanding of calcification chemistry.
One slight problem I've encountered, which I've noticed is worst when I've waited a long time between dosing, is that when I add the sodium carbonate, I get precipitation immediately of what is probably calcium carbonate:
Never seems to harm any inhabitants, even when it accumulates on top of corals. The precipitate forms soft, snow flakes which dissipate after a couple of minutes. They must look and feel like food particles, because my fish hunt them as voraciously as they do frozen mysis. My hairy mushrooms get fooled as well, and usually one or more of them will close up as if capturing food. Pretty funny, but so far no ill effects.