One problem that can cause many others is unbalanced pH. As someone else suggested, go to a pool store. They can do a whole series of tests, often for free, including for phosphate. Cyanuric acid level is also important.
There are non toxic chemicals that will remove phosphates, but the only way to remove nitrates is by draining.
Yes - go to a pool store. One problem with just dumping chemicals in is that you don’t know what you’re doing to the overall system.
pH is important, it’s the balance between acid and base.
Too much acid, the water starts eating your pump equipment.
Too much base, the water starts eating the concrete.
(Maybe the other way around)
Too much chlorine, it stinks.
Not enough, it stinks.
It’s a bit of a balancing act. Treat it like it’s a pool or a spa and you’ll do well.