Fish Dying One By One, Can't Diagnose

Sorry for the biggish post. Tons of people have dying platys, but I haven't found a post with my particular set of circumstances.
I have a 29G community tank that has been running happily for one year. There are a couple kinds of tetras, female bettas and a lot of sunburst platys, an original pair and then about 10 or 15 kids of varying ages. Heavily planted, I inject CO2, with drop tester and math methods indicating I am at 19 of the recommended 35ppm I dose PPM, 6ml Macro .6ml micro.
Then the following sequence occurred:
Day 1: removed gravel and added 2 in of Eco_complete including the nutrient liquid in the bags. Left fish in tank. Turbity was significant for about one hour. After 2 hours of filtering water was close to clear. 75% water change, and parameters were PH 7.2, Amm 0.5, Nitrites 0.0, Nitrates 10, GH 14, KH 7, Phos 0, Temp 75. These parameters are the same as the year of happy living except for ammonia, which had previously always tested at 0.0 to 0.25.
Day 2: Ammonia still at 0.5. Water clear. Fish seem fine, except one betta is sluggish, begins to show white fuzz in patches along body. I start to try and diagnose that.
Day 3: Betta is worse. I dose with Pimafix, but betta dies that night. All other fish seem ok. Ammonia back down to 0.25.
Day 4: Fuzz/fungus never returns on other fish, but Platys begin to die one by one for next 5 days, one a day, sometimes 2. I have lost 8 so far. They seem fine and energetic and have appetites, but then one will slow down, go still either among floating plants but occasionally near bottom in a sheltered spot, and die there.
Day 5: Last betta heads towards death, floating at an angle among plants, still barely breathing. I notice a film on the surface of the water, I bet it had been there since the gravel swap. Read that biofilm can decrease gas transfer. I take paper towel sheets and drag then quickly across surface of tank. Most biofilm comes up and surface looks clear again.
Day 6: Betta seems to recover, swims and eats again. That night, another platy dies.
Day 7: Another platy dies. I have no idea what's going on. Post on Fishlore.

Anyone have a clue for me?

 

Throw a bubbler/air stone in there to helo break surface tension and oxygenate the water. Its possible with the film on the syrface and dosing co2 that the numbers may have climbed in the water and overwhelmed the fish?

My only other thought was that somehow the ecocomplete brought in something that infected the fish. White fuzzies on the first betta you lost sounds like columnaris but it doesn't sound like any of the platies were showing signs of columnaris before passing?

 

Columnaris seemed the best guess from the web, so I bought the Pimafix, but the betta died soon after the first dose. And yes, it baffles me that the Eco-Complete would have brought in a fungus that only affected one fish, and not even the other betta, and no sign of it on any platy or tetra. CO2 was at 19.5 ppm last night according to two tests, which the web tells me should be totally fine for a high-tech planted tank. Is there any good test for dissolved oxygen?

 

It is very odd, I have not known columnaris to go after only one fish in a tank, especially when there is more than one soecimen of the same species. They maybe don't all show signs at once but columnaris tends to spread to most tank mates and show eventually.
I've used eco complete myself and never had any issues with it bringing anything in to my tanks. Granted I've also only ever added it in when setting up a tank with plants and inverts first, no fish, so no idea if I could have had something and it just died out before a suitable host was put in the tank.

I'm not sure of a dissolved oxygen meter that isn't going to cost you upwards of $200+. They aren't cheap. If you want such a device they have them on amazon but again, they aren't cheap.

 

What about something like this to test oxygen?

 

I’m reading about it now, and it seems like the consensus is that in a tank with low oxygen, the fish would either be staying near the surface and gasping or hanging out at the outlet of the filter. None of that is going on. All the Tetras are zipping around happily, and the Platys are doing the same. Even when the Platys die they don’t seem to head to those areas. Baffling. I’m thinking that replacing the substrate must have shocked the tank in some subtle way that doesn’t show up in water tests. And that the Platys are particularly sensitive to it. Maybe that fungus was something uncommon that does not spread easily? So far today, no particular fish is acting sick.....