I've tried before, even took my water to Manhattan Aquariums and they tested for everything. I have a mild case of green hair algae, and my hard corals slowly lost color and died over many months, some covered with algae and others not. I recently started losing zoas too; I just noticed there were less and less each time I looked. I have inverts so I know it's not copper, I have my own RO/DI filter, and I do weekly 10% water changes with Tunze Excel salt. Just checked and my phosphates are at .45PPM according to my Hanna Checker, and <.1PPM according to my API test kit. Calcium is 360 according to my API test kit, and I haven't dosed or done my usual water change in 3 weeks (doing one tonight). pH is 8.2-8.4 as per API, no nitrates/nitrites/ammonia.
My latest theory is my light is either too strong or too weak, or the GHA in my sump is cycling back into the tank and killing things. I talked to a guy who's gonna stop by for $60 and check my light with a PAR meter, and today I removed all the algae from my sump and switched off the light. I guess we'll see...