Mysterious Fish Death

[rikker4u]

I have a 37 gallon reef tank with about 80 pounds of live rock with some corals. It's been established for about five years. About five days ago I found my cleaner shrimp dead on the bottom of the tank. I pulled him and did a four gallon water change. The next day three PJ Cardinals died, two of which were at least two years old. I did another four gallon water change. We then lost our starry blenny and now our two clowns. The corals seem to be dying too. Hermit crabs are dying. I haven't done anything new in three weeks when I introduced the blenny and some crabs. The only things left seem to be a sandsifting starfish and some snails. I noticed a bright green, almost neon, algae had formed on the sand. That's the only thing new. I ordered a testing kit, but haven't received it yet.
Does anyone have any idea what could have happened?

 

[woody390]

Unfortunately would probably need your test kit as most likely one of your parameters is out

 

[Sniperhank]

Old tank syndrome. Continued buildup of crap over years and years will lead to an increase of deadly parameters. The shrimp dying would have been an indication of this as inverts are more sensitive to fluctuations in levels and spikes than fish are. I would do a large water change and try to siphon out as much crap from around your rocks as possible with some airline tubing. The algae that is forming is feeding off of the nitrates from this buildup. Might also be a type of cyano.