Aquariums: Community tank problems!

Any advice or suggestions are much appreciated!! Tank info below. This is our first guppie/molly/swordtail tank.

Just came home after 2 days to find 6 fish dead in our community tank. Looks like a nitrate spike.

I have had very successful aquariums over the 15 years. Never had issues that I’m having now. This is my first community live breeder tank I’ve ever done. Also first tank I’ve let my daughter and wife decorate with artificial plants and decorations. I have always used large driftwood and live plants in other tanks.

Have had very little success adding fish.. right now we probably have 12-15 fish. we buy 2-3 guppies or mollies every 2-3 weeks to add to the tank and hardly ever have any survive more than 1-5 days. We have an extremely small success rate for some reason most of them die the next day or a few days after adding. I drip acclimate them and do everything by the book like I have for years. I have no idea what’s going on and now this. Never had this many die this close together (over 2-3 days).

Now fish that we’ve had for over a month are starting to die here and there this week.. not many, 2-3, but still.

We set our tank up 6 months ago and had trouble cycling it and let my wife rush adding fish at around 3 weeks after setting it up for our daughter. Lesson learned.

Could the nitrates be from the decaying fish from the last two days? Ppm has never been this high. It’s normally next to non.

Did a 30% water change last week before this happens. I’ve been only using slime coat for water changes. Used prime and stability when we first cycled the tank. I still have prime and stability available for use.

What ppm does this look like? Is this high for this tank?

Should I do a water change again? If so what %? Could the water change I just did be the reason this happened?

75 gal tank – Fluval fx4 with aftermarket intake and spray bar (very light flow/current) – 78 degrees – mollies, guppies, and swordtails.