I doubt CO2 overdose killed them. CO2 doesn't displace O2, so they would not have suffocated.
And pH decreases related to CO2 injection don't affect fish the same way as pH changes related to alkalinity increase/decrease.
Not saying its impossible, but I suspect something else...maybe the DIY mixture flowed up the tube into the tank. You say you had a hose clamp....you are aware that you ABSOLUTELY CANNOT restrict the flow of a DIY CO2 unit right? If you clamp the end, the 1.5litre bottle continues to produce CO2 until it pops the clamp, or the bottle explodes.
if you had it clamped, it likely built up enough pressure to pop the clamp and then the sudden release surged DIY mixture into the tank, killing them via poisoning.