having a hard time with SPS and chalices

[BilboB]

I've had my 55g coral/fish/inverts tank for about 6 months now and although the softies and Euphelia are doing well, all the SPS and some LPS (chalices mostly) slowly die. I'm trying to figure out the problem.

I have Nicrew Hyperreef 2 lighting, which produces ~150 PAR just under the water surface and ~50 PAR at the sand. I use 2 MP-10s for flow. I use an in-tank skimmer and in-tank filter unit (both from Tunze) since I don't have a sump or back overflow area. I change the filter foam twice a week and keep bags of activated carbon & PhosGuard in the filter unit. I do a 40% water change every month and a 20% change every 2 weeks in between, using Red Sea Coral Pro salt. I use a Tunze ATO running Kalkwasser to replace the lost water and calcium. I run a high-power UV unit every night, using a small pump inside the tank. I even add 5 ml of vodka to the tank every night.

I test my water every Friday and the numbers are fairly stable: around 0.2 phosphate, 20 nitrate, 8 alkalinity, 420 calcium and 1350 magnesium. Water temp is 78 degrees and pH is 8.2.

The only thing I can think of is that alkalinity might vary day to day, since I don't do daily dosing (I occasionally add bicarb/calcium/magnesium if the Friday levels are low). But that's just a guess.

Any help would be appreciated.
Rob

 

[rayadog]

No expert but my first thought was low par or young tank. Idk if the young tank idea has credence that I can speak to. I think if you can touch some spots of 200 you might have better luck? I manually dose alk every night so my alk likely sawtooths just an idea. Some days I forget…

I’m assuming if you have 150 at the surface the par at the rock is lower than that.