High nitrates or faulty test drops?

[Cermet]

You are in a bad situation; those fish will push the nitrites through the roof and water changes will take a while since the substrate is packed with waste, as well. As for the nitrate readings, from the algae cover, your nitrates (and phosphates) are high. If you are getting any ammonia reading and since you have well water (hence your filter was not harmed), the only conclusion is that you are under filtering!

Two choices: continue the very large (50% at least bi-weekly) water changes and get another filter on the tank.

Or, add a algae scrubber system - it will eat the nitrates (and phosphates) and remove any extra ammonia that the filter does not handle. Your water changes can be reduced a great deal (to just a 50% weekly would most likely work well.)

This will solve the waste build up (nitrates are a toxic waste at those levels) but the fish in that small tank is an issue. A 75 gal has a identical length and height but a greater deapth, so that might work. A 90 can be identical to a 75 gal except much higher. These might work for your situation.