Looks like green hair algae and I have seen worse. You will beat it with a little patience. That tank will look great in no time. Someone mentioned a 30 gallon WC. I previouly mentioned a 10-20% WC because I didn't think you had live rock and I felt a large WC might be too much at all once. If the rock is in fact live rock, then go for a larger WC and clean the rock off as best you can with the old water. Also, live rock does not require light to be live. However, please keep in mind that you don't want to swing specific gravity too much too quick. I'm sure you'll get the tank back on track. Just ask anything you need. Good luck!
Oh, in regards to the need for a pH buffer. Chances are there is going to be a difference in pH between what is in the tank and your fresh water mix simply because of the presence of algae in the tank and the process of photosynthesis. If you are going for a large 75%ish WC you might want to consider trying to match up the pH. If you are only looking at 20% or so WC, I would say that a difference of +/- 0.4 would not necessitate tweaking pH and just do the WC. I never use a buffer but I use a consistent water source.
Oh yah I should have mentioned taking out the fish and place them in a bucket with a heater and power head.....acclimate them like you just bought them after that huge change I recommended. The difference in water quality could be enough to stress them even more...and the big water change is already a stress...
Good call on PH because sounds like this tank lacked proper maintenance so PH could be low and fish accustomed to a lower PH vs higher. So acclimating the fish is best ....
I thought it was worse than this...
Good old siphon out water to bucket scrub a dub dub needs to be done....
I guarantee those nitrate and phosphate readings will be false...because there is enough in there feeding on it...for instance if its 20 nitrate it probably is much higher but they are feeding on it keepiing the readings a little lower.
Good stuff we know what we are dealing with now.
Plan of attack:
Do at the least a 50% water change....
Scrub those rocks in the bucket of siphoned water...
Hand pull all you can out.
Every week do 10-20g water change it will put a stunt on the growth.
Prevention is all you do from here:
Keep nitrates down...keep phosphates down.
On a budget:
Buy them a phosban reactor and use phosban ...that will remove phosphates in the water....that is fertilizer for algae.... best purchase I have ever made and I constantly recommend this.
And yes use a PH bufffer because on initial break in of Phosban it hits the PH.
From here out if you buy that it's all about nutrient import control...explort handled by the water change. Feeding lightly.... Formula 1 you mentioned is good ...but mysis shrimp from time to time is good to. They need a varied diet to be strong like ox.
