Dead cheato, green slime

[Pilot_rue]

howdy from Texas
I am having trouble growing chaeto in the sump on my 126 gallon tank. It is well established tank with 9 small-med fish and one med-large trigger and is just at 1 year old. As I try to add chaeto it last about two weeks then turns into a green slime. Causing the water to start fogging. I even tried using brightwell chaeto grow.
but yet I am growing it like weeds in my 50 gallon mix reef tank.

current water conditions on the 126
temp 78
PH 8.23
ammonia 0-.25
nitrite 0
nitrate 5
phosphate 4.0. (yes this is bad )
carbo hardness 9
calcium 440

any suggestion?

 

[Subsea]

Symptoms indicate limitation of trace mineral. Obviously, that is what chaeto grow is for. If you really want to know then get an ICP test. For certain, I would dose and test for iron.