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I would do a water change, gravel clean the substrate, clean the filter and glass, then add salt. If the red line is bacterial, the sooner you start treating, the more chance of success.
I got a good amount of the algae cleaned off and did a large water change so I guess I'll add the salt rn. There's still some algae and stuff but I ran out of time cause the water level was getting low.
If you used a tablespoon from the kitchen, use 2 heaped tablespoons of salt for the 20 gallon tank. If there's no improvement after 48 hours, add another 2 heaped tablespoons of salt.
You can try a broad spectrum medication that treats bacteria and fungus. Methylene Blue does both and is pretty safe but it wipes out filter bacteria and stains things blue so use it in a separate container. If that doesn't help then maybe an antibiotic but I don't know which one or what is available.
You can leave the salt in the tank while you use Methylene Blue or other medications.