Red Slime!

Hello there!
I work at a children's hands-on museum and we have large (200+ gallon) touch tanks that currently house southern/atlantic stingrays, horseshoe crabs and puffer fish in a separately connected tank system. We have slowly been developing a red slime algae problem and its getting worse and worse. The red algae is in a smaller, thin, side tank housing the puffer fish (a porcupine and a dog face) not with the stingrays. I have taken the rocks/fake plants out of the tank and scrubbed them individually but I haven't seen an improvement.
I have been doing some research, and it says pH can have an impact as well as phosphates. I do water changes about every four days. I do water kit testing every other day. Our nitrates for the tank are at about 80 but that level has remained consistent from before the slime even appeared. What phosphate level am I looking to achieve to rid the algae? Do I use sodium bicarb to raise pH? I'm thinking pH should be around 8.2? Currently its at 8.0. Our UV sterilizers are no longer functioning and our budget currently doesn't have an allowance for a new set so I'm thinking that will be a major cause of the growth. I was reading how water flow speed has an impact on algae growth but with the position of the tank where it is I don't have outlets to place additional wavemakers in the tank.

I'll really appreciate any advice I can get! Thanks so much

 

Heres a quick photo. The long thin tank is where the algae is.

 

Welcome to 3reef.

What type of lighting system please?

Do you just need a new bulb for the UV?

An elevated nitrate level with the fish you house is to be expected. I would still try and bring that level down some, I image your phosphates are elevated as well. Consider some way to export those nutrients?