What is considered an unsafe level of nitrate?
Mines was sitting between 20-40ppm for a while with no setbacks. That being said you should still strive to keep it as close to zero as possible. You get more leeway with your nitrates in a fish only system, reef not so much. I finally added a HOB
skimmer and it took my nitrates down to less than 5ppm.
- How long should I wait before introducing more fish?
Seeing as you added 3 fish at one time I'd wait a few weeks before adding anything else so you don't overwhelm your system.
- Is there any fish I should avoid mixing with my current fish?
Definitely stay away from more tangs. Live Aquaria has a compatibility guide that's worth checking out.