Yes sir I was afraid for that.
Your test come back 0.0 but it doesn't mean there isn't any in the water column cause the algae is taking it before you cam measure it.
Make sense right

yet you have some Po4 and No3 in there that your algae is thriving on.
So if I read that correct no CORALS??
Change schedule for lighting to noon to 10pm.
What kind of lights we talking here?
Pics are great and tell me thousand words.
You media ain't doing anything other than removing big particles from your water.
Now if we talking like a GFO media than yes.
I hope not if they are Tangs as that's like putting 3 sharks in a nano, tangs need large tanks starting from 120 gallon and up.
Send a picture from one of the three fish.
I just looked at the FTS (full tank shot) and we dealing with 3 yellow tail damsels not tangs

.
Ok what we can do is we need to check the new mixed salt water that goes in on for a water change on Po4 and No3?
What kind of test kits you using?
If you mixed it your self what kind of water you using, faucet or RODI and if so what is the TDS from that water?
Also reduce the light but I will ask you to go three days of total darkness.
I know this is boring but see it as a very stormy day on the reef.
Turn lights off and cover tank with a dark sheet or towel.
Algae won't grow without light and three days might be enough to kill it all.
In that period we don't feed the fish or peek but leave everything running as pumps and filters but not the lights.
After three days remove what you have over the tank to cover it from the light and turn on light on a reduced schedule.
Use some Prime as directed on the bottle to spike the bacteria load (any kind of bacteria for salt water works)
Also I like to point out that you might need to remove some fish and my suggestion is 2 of the blue fish as they in later time will go after each other and only one blue will survive that struggle.
Don't put any more fish in there after wards.
Clean filters more often and make sure skimmer is working properly to remove nasty proteins from the water.
Keep up the water changes but remember I need to know the test results first from your new mix salt water as stated above.