Coral reef problem

[mfbs1998]

Hello there!
So I started my 20 gallon saltwater tank about 3 months ago. Right now, the only thing I have in that tank is 2 clownfish, a yellow watchman goby, and some inverts. I have started to do coral just a few days ago with xenias, zoas, and a frogspawn. All three of them opened up immediately when they were put in the tank but just a few hours after that, my xenia is on the brink of death, zoas are closed up, and the frogspawn shrunk. I don't know what is going on so can anyone help? All of my ammonia and nitrites are all zero but the nitrate is low. My pH is also at 8.4, I have a canister filter running that is rated for 55 gallon, and a high output t5 light with two bulbs

 

[Diesel]

Did you test for phosphates?
A canister filter can be phosphate reactors if not cleaned enough like every other day.

 

[mfbs1998]

Yah. Phosphate is reading at 0 so I don't know what is going on. The only thing I have in the filter is some bio balls, polyester filling, and carbon

 

[Diesel]

It might be that your tank is to clean with No3 and Po4 at 0,0.
How long has it been like that?
Did you dosed anything in the tank lately?

 

[MasterBacon]

Pictures?

 

[mfbs1998]

I haven't done anything much lately. Ever since I started the tank the only thing I have been doing is weekly water change ( 25%-50%) and feeding them twice a day. I don't dose anything for them since I thought that the light will be strong enough

 

[mfbs1998]

Here is some pictures

 

[ahiggins]

Oh man....that last one is pretty bad especially for it being one of the best growers/weedish type.
With your water changes, thats a LOT 50%?! also what salt do you use? I know when I used coral pro salt from red sea, it made up to super high alkalinity. I never noticed until something died.