I responded to your PM message last night, but wanted to post here, too.
Your tank is looking really good! I know it can be discouraging when we hit bumps like this, but don't give up - things will settle down for you soon. *hugs*
It is unusual to have nitrIte in the tap water, as Hallyx pointed out - I hope you aren't drinking the stuff! If I were you I'd contact the local water supply company about that - it isn't safe for YOU, much less your fish!
With that being said, don't worry about overloading the bacteria. There is more than enough surface area in your tank to support the bacterial colonies needed to handle the bioload of your fish along with this level of nitrItes - but it looks as if a bit more time is needed for your tank to fully cycle, and those bacterial colonies to establish.
Sometimes the most important thing with tanks is to be patient and allow nature to take its course, and I could be wrong, but this looks like one of those times to me. Again, as Hal mentioned, you can try adding a bottled bacteria product like Tetra SafeStart (which should be easy to find in any fish shop) to possibly help in speeding the process up. Apart from this, as the others have said, all you really can do is be patient, continue to do your water changes, and be sure to add Prime once every 24 hours to keep the fish safe as things settle down.
* IF you are showing a reading for nitrIte in the tank
water BEFORE your water change, you may want to double dose enough Prime for the entire
75 gallons (if you aren't already!) Continue to add the full amount every 24 hours like clockwork, even if you haven't just done a water change, for as long as you are testing nitrItes in the tank water.
* When you do your water changes, avoid disturbing the surface areas of the tank or your filter during this time - you want to do your best not to disturb the areas where the bacteria is growing!
* I would suggest possibly limiting the amount that you're feeding your fish, and feed them only once every other day until things settle down in there. The problem here is in the tap water, but it can't hurt to try to lower the amount of waste that goes into the tank until the levels even out
* The nitrItes may still show up on the test, even though they are locked down with the Prime - watch your fish carefully for signs of distress, and should you note anything out of the ordinary - like the gasping you mentioned earlier - do an immediate water change, adding a fresh dose of Prime.
A couple of questions for you. . .
1. How long has this tank been set up?
2. Did you cycle this tank before adding fish, or did you start with a fish-in cycle?
3. I'm confused about your stocking - do you have fish somewhere else that you moved from this tank to keep them safe? Or are all of your fish living in here now?
I know how confusing all of this can be! You're doing a great job, and getting a lot of good information here - you'll be through this soon, and things will get much easier for you! I'm not positive that all of your questions have been addressed, so I encourage you to please repeat any that we may have missed! Keep up the good work - it looks like you're on the right track, and doing great! ^__^
I hope this helps you out a bit - good luck, and keep us posted!
(I'm still a beginner myself, so if I have posted anything misleading or confusing, I ask that those with more experience always please feel free to correct me! I know I have a lot to learn still!)