I'm guessing the white crap is "dead fish fungus". Remove the corpses and siphon out the residue and it should go away. Was the filter running when you got home? Its possible the slime (some kind of algae, most likely) clogged the filter media enough to block the flow.
I can only speculate but I'd guess that you hit the nail on the head with CRASH. The CO2 cause a sudden drop in pH that either did in some fish or wiped out a lot of algae. The other possible root cause is your filter not working because it became clogged, that combined with the CO2 would mean the fish would have trouble getting enough O2 and some could have died of it. Everything else is the result of having a lot of dead, rotting stuff relative to the size of the tank. You must have had an ammonia spike since you had a nitrite spike when you tested. The cloudiness is bacteria multiplying to take advantage of the extra ammonia and nitrite to eat, the white string are fungus feasting on dead fish, and the slime is either a rapidly growing algae or dying algae that sloughed off the surfaces it was clinging to. The gasping for breath was the result of ammonia/nitrite poisoning which may have killed more fish than the initial pH drop.
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how can i get rid of the slime floating in the water
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If its floating on the surface skim it off or soak it with a paper towel, or better yet, drain the tank and rinse the the substrate.
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what can i do to save my fish?
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You've done it, the best cure for poisoning is clean water. You may have a few more loses but if your fish are acting fine, most of them will recover. Consider buffering your water with baking soda to prevent this from recurring. Always watch pH and hardness when adding CO2. I really think that if you refill the tank with clean water, use Prime or Amquel+ to lessen the affect of ammonia, your tank will have a little mini-cycle and bounce back to normal quickly. You may wish to leave some of the fish in the little tank for a while, but don't leave all of them in there or the overcrowding could hurt them. Watch the ammonia and nitrite in both tanks for awhile.
I always suspect water quality over disease when several fish die at the same time with no prior symptom. However, if you find white stuff attached to still living fish it could be columnaris which kills rapidly. If you see this or other symptoms, start reading the disease sites. Otherwise, it was "just" water quality and your tank will be "fixed" by clean water and a little time. Even rinsing in warm tap water will not totally destroy the filter's colony, so it should bounce back and you won't have to "re-cycle" the tank from scratch. And if slime was restricting the water flow it needed to be done.
Things like this are why I tell people to get a bigger tanks and/or fewer fish than you want. It gives you some insurance. But I don't think your 15 was overstocked, its just that smaller tanks are much more prone to pH swings (and hardness swing and temperature swings). Please think twice before trying CO2 in the 6.