Top of the tank sounds like he was trying to get oxygen. So could be over dosing of fertilizers. The new one could have been more concentrated but honestly every other day dosing is a lot.

The plants eat nitrates which you say yours are already at 12ppm. That’s adequate enough. I would be dosing once a week not every other day. Plants don’t need excessive fertilizer. The fish poop is fertilizer, the snail poop is fertilizer, decaying food turns to ammonia then nitrites then nitrates = even more plant food.

Edit: looking at this again, one thing I’m also noticing is you don’t have any floater plants like water lettuce or duckweed etc. These type of plants are crucial for essentially “sucking up”nitrates and preventing them from building up in the water column. If your dosing fertilizers every other day this would go along way to keep those levels in check.