2 fish 4 days

Yeah, I am going to say low oxygenated water combined with a stressful 1.5 hour acclimation in water that was probably even less oxygenated did the fish in. I brought up your low pH in a previous post, oftentimes low pH can be caused by not enough water movement resulting in low oxygenated water.

You need more movement in that tank than the 234 gph return pump. In my 29 gallon I have two maxi-jet 1200's pushing 300gph each and pointed at the surface, I have a Koralia 1 pushing 400 gph and my skimmers pump at 300gph.

Get yourself a Koralia 1 or 2 and get some more water flowing, especially at the surface. Wait a week, get a new fish and drip acclimate it at 2 drips per second for 30 minutes I bet you will be golden.
 
Check your pump suction/discharge for any blockage also.

Air pump (place in tank) will b your easiest/cheapest way to c if oxygen was your problem while keepin that pump running in there.
 
Good luck.

I feel certain that if you get oxygen in there via air pump or even better flow as like koralia/maxijet and you will be good to go. check local for cheap/free maxi's. I know myself i ve thrown so many of them away. walmart like $20 or so.

A stream of bubbles look cool in the tank anyway. imo
 
the bubbles poping on the surface are gonna make mess, its acting like a skimmer w/o a collection cup. Go for the powerhead, much more useful, and REALLY get a skimmer man!
 
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