Yes, CO2 at high levels will wipe out any fauna you have in the tank. The plants though, will appreciate it.
Do you have a tank where you can relocate your fauna while you rework the tank? This might be helpful in reducing stress.
You need another check valve. Get a package of Rhinox if you can. Well worth the money.
Yeah, I really hate losing fish... I decided to reduce the bubble rate on the counter for now, seeing as there aren't that many plants in the tank at the moment I probably don't even need that much CO2. I've also changed the timer for now so that CO2 starts only 1 hour before the lights turn on and shuts off 1 and a half hour before the lights turn off. I think I'll see how long it takes for CO2 to get to the diffuser tomorrow and adjust the start times from there. I'll also get round to taking a pH reading tomorow before the lights turn on (should have done this from the start).
I do have another tank actually but I don't think I'll be putting the fish in there, catching them will be a huge pain in the ass. I'll probably put the fish in a big bucket before I tear out stuff.
I ordered a bunch of plants from two shops and one of them will be throwing in a cheapo check valve for free! I think I'll use that for now and then go out and get a better one once we're allowed to go out again lol. For now, can't wait for my plants to get here and also for the weekend to get here so that I actually have time to work on the tank... They will probably get here just in time for the weekend.
Fish wise, I'll probably get some more ricefish to replace the ones I've lost... but since I've been losing the ricefish at a fairly alarming rate, I don't actually know if the ones I currently have will make it till the end of the circuit breaker. Maybe the water I have isn't that great for them or something. If I do lose them, I guess i'll get 1 or 2 more sparkling gourami to make a group of them.
EDIT: I thought i might as well document a few things I observed while using the CO2 Art regulator. Even after the solenoid closes, CO2 bubbles still come out from the diffuser. Might be normal if its just CO2 leftover from the tube but I noticed that I still get bubbles coming from the bubble counter, although at a very slow rate. This continued to happen almost 1 hour after the solenoid shut off.
The solenoid is definitely closing fully. I tested by unplugging the CO2 tubing from the bubble counter (don't want to gas the fish again) and left the needle valve almost completely open. I got a load of bubbles, but the moment the I turn of the power and the solenoid shuts, the bubbles stop almost immediately.
I'm not sure why this is the case but it seems that it takes a really long time for the CO2 to stop at the low rate i'm dosing, but if I leave the needle valve open a bit more, the bubbles stop pretty much immediately when the solenoid closes. At least I know the solenoid isn't faulty or anything... but I guess it means I have to time the solenoid to power off probably 2 hours before the lights switch off. Ah well, I guess i'll get the hang of it eventually.