Newbie 75 gallon

[gregzz4]

Are you using a GFCI? And if you are, are you using a ground probe? I gather you are not using either. You would not get a shock if you were.
If you use both, next time a powerhead fails it will shut off. Do not use a ground probe without a GFCI. Doing so will send stray current through you!!
I wonder if stray electrical current in your water has been an issue.
Hopefully things get better for you now that you have removed the second bad powerhead.
Run the skimmer wide open for a few days, after you first clean it with vinegar, then slowly try to tune it. It will probably not do very much right now as you do not have fish poop.
Unless you are going to source some mysids or copepods, do not waste your money on pods.
Tigger pods are only a quick snack in our tanks.