corals not opening-help!

I noticed yesterday my corals(all begginers softies btw)were looking kinda blah,they just wasn't out all the way.I only test the basics,but they were ok
sg 1.024
ph 8.1
nitrite 0
nitrates 20ppm
temp 79-80
cal-420
I did a 20% water change(ro/di) with reef crystals(same brand i allways use)and it dropped my nitates to around 10ppm.This past week i had a little cyano show up(working on it)and a 2nd diatom bloom,which i mentioned in an earlier post.But it looks worse than it did yesterday,the corals are more closed,fish seem fine.I've also noticed quite a few of my dwarf ceriths are lying on there backs but when i touch them,they retreat in there shell(so there still alive)but i'm stumped.Can i have ammonia without showing any nitrites??The tank is going on 8 months old.I did increase the flow on one of my powerheads today to see if that helps and added a packet of phos-x phosphate remover(little pouch,says it treats 15 gallons,i put it in my filter)Any suggestions is greatly appreciated!!

 

How long did you aerate/mix the saltwater before doing the waterchange?

 

About 3 hours with a powerhead and heater,water was crystal clear(fully dissolved)and same temp as tank.

 

Doing a waterchange and adding a new product like phos-X might have been too much change at once. Since you can't pull out the new water, why don't you pull out the phos-x and wait an hour or so and check it again.

 

where is the water coming from? You may have a bit of ammonia in the replacement water causing water quality issues.

 

I was seriously wondering about the phos x,should i do another water change after removing it?.I buy my water from wally world by the jug,not out of the refill machine.I've been using it for about 5 months now.

 

You may want to test it just to be safe.

 

do you mean for ammonia??I could definatly be wrong,but with the age of my tank shouldn't i show some nitrites as well?? I don't have an ammonia test kit,i just use those dip sticks(i know they're not the best,but it's a low budget tank)

 

I was just looking at the tank and theres more ceriths than i thought that are lying on there backs belly up than belly down,but when i poke em with a stick,they are still alive.Are they dying off?? and should i start plucking them out except for the healthy looking ones?

 

I had this problem with my zoas being closed and my snails dying, it was b/c my salinity was 1.035 while my swing arm read 1.025. I lowered the salinity and the zoas re-opened.
So how do u test salinity?