It's quite murky, so I can't tell if they're there or not.

I just checked pH and it looked like 8.2 (ouch) so I put in some pH
reducing salts. It's supposed to be 7-7.6, right?

It's about 500 gallons, by my estimate. At the moment I only have a
little pump that circulates water down a very small waterfall. Should
I get a filter as well?

thanks