What is wrong?

[Sk8r]

often people get lulled into a false sense of security by dealing with a fish store with the same salinity as they have in their tank at home: they dump inverts straight in with no acclimation, no problem. Then policy changes, new ownership, new store, new source, who keeps their salinity different---and everything you buy dies. A refractometer test of water in the bag and in your tank will tell you the difference you're starting with. Acclimation is mostly about [but not solely] salinity and temperature, and of the 2, salinity is the big one. Make sure you do a gradual change until the salinity difference is within .001. This is why any instrument BUT a refractometer is risky, and may cost you bigtime sooner or later.
Having your own ro/di unit and watching the TDS of that water is the first step in controlling algae. They can test for phosphate until they're blue in the face, but it won't show if it's 'bound' in algae. If you have algae, you have phosphate, no question. Algae doesn't exist without it. A lot of phosphate comes in with non ro/di water, or ro/di where the filter is getting tired. It also comes in via dry fishfood. And algae, if you import any.

[otrlynn]

Also, putting new fish into a quarantine tank would let you observe them to see whether they are as healthy as you hope the are. Observe the acclimation procedure as you put them into quarantine, and when you are ready to go from quarantine to your display tank. From what I have read on this site, it seems like quarantine should be at least two to four weeks. This should help you figure out if the fish you are buying are unhealthy or your display tank conditions are the problem.