Dean,
Please stop referring to pH using the units of ppm. The ppm unit is an acronym for "parts per million" and has no relevance to pH which has no unit. It is just the logarithm of a simple number. That simple number is the number of free Hydrogen ions (H+) in the water. The calculation of pH goes something like this: pH = log10[H+]
Knowing that the pH is 6.6 you can solve the equation for the H+ count:
[H+] = 10-pH
[H+] = 10-6.6
[H+] = 2.5 X 10-7
That's the number of moles of Hydrogen so to get a real number multiply by the molar constant 6.02 X 1023
So [ 2.5 X 10-7] x [6.02 X 10-23 ] = 150,500,000,000,000,000 or, about 150.5 million million Hydrogen ions.
OK, so then your pH dropped to 6.4, so doing the same calculation, it can be seen that the number of H+ increased to 239,660,516,673,205,344, or about 239 million million Hydrogen ions. That's only a 60% increase.
Ideally, you would want the number of Hydrogen ions to increase at lights on from 150.5 million million to about 1,505 million million. That's a 1000% increase, and that would show up as a pH of about 5.6 at lights on.
The increase in free Hydrogen ions is a direct indicator of how the CO2 is dissolving, because when CO2 dissolves, a small amount gets converted to acid and this acid, by definition liberates Hydrogen ions (H+) into the water column.
So this is part of your problem. I specifically did all this tedious calculation so that you get a better feel for what the heck the pH is and what it is that we are trying to accomplish. Taking a pH reading tells you what the rate of change in the Hydrogen content is, which in turn, tells you what the rate of change of CO2 in the water is.
The goal is to change the pH from the "pre-gas on" value to some value approaching a -1 unit pH difference. This is not always achievable without annihilating the fish. It's a very tricky business, but at lights on is THE most critical time of the day for CO2. That will make or break your tank. After about 4 or 5 hours of gas the plants really do not care so much about CO2. So it's possible to increase the injection rate and then to turn the gas off very early to avoid toxicity. When you turn the gas off early after a high injection rate, the residual CO2 content will feed the plants until the end of the photoperiod. Also, flow and distribution are primary factors, so if you want to avoid toxicity then you have to make sure that you have good flow rate and distribution techniques. That way you don't have to drive the pH so low because the flow and distribution will carry the CO2 more efficiently to the leaves.
In any case, forget about film and sponges for now. They are symptoms, not causes.
Fix your CO2/flow/distribution first. Make small incremental changes to the injection rate and/ or turn the gas on earlier while measuring the pH changes from gas on to lights on. Obviously you want to avoid killing the fish, so do this on a day off when you can be home to monitor the tank.
Cheers,