Overstocked ?
 

Hey all just a quick question, I have a 240 gal that has been up and running for about a year now, lately my cyano and gha is getting out of hand. And curious to see if over pop could be my issues. Here's the lineup
Sailfin tang
Achillies tang
Yellow angel
Koran angel
Blue tang
2x clowns
Blue line angel
Fox face
Crosshatch trigger
I do weekly 20% water change with RO, and get decent skimming.
Thanks again

How big are these fish? Do you plan to keep them in this tank indefinitely or do you plan to buy a bigger tank to house them as they grow?

Fox face 5in
Blue tang 4 in
Sailfin 5 in
Clowns 2 in apiece
Crosshatch 7 in
Koran 3in
Blue line 3in
Yellow angel 3 in
Achillies 4in
Queen angel 6 in

I plan to keep them in this tank

At the sizes they are now, you're not overstocked, but I think you will have a tough time keeping them all in that tank permanently. Many of them will double in length and probably quadruple in weight. That will be a lot to ask of a 240-gallon tank.

The crosshatch and foxface are probably your biggest bioload contributors and this will only get worse as they continue growing.

How much and what are you feeding, any other changes lighting, flow. Are you dosing anything have you checked phosphates? Do you know the tds reading of the ro water

I feed once a day with pellets, no dosing, and never thought to tds my water but the ro unit has pp reading of 0. Lights are 9 hrs

Do you have a sanded?
Any detritus piling up behind the rock work or in the sump?

To the OP about overstocking. I'm no expert but I've seen a lot of tanks. But really have any of you actually seen fish in the home aquarium actually reach the predicted size that liveaquaria or other sites predict? So IMO it's another issue rather than overstocking. A little more tank and maintenance detail and maybe we can get your issue resolved.