Clowns Died

I have a 29g nano cube, I have live rock, and used pre mixes salt water bought in jug. Been running about 2 1/2 months, live rock was completely cured before being used in tank.
I started getting red slime alge and didn't want to treat with chemicals so I decided to rearrange the live rock to increase water flow and stop feeding so much.
I rearranged last night and did a water change of about 20% to try and vaccum up all the red slime. There was a huge influx of air bubbles that filled the tank when I took out one of the rocks and the water level dropped.
The clowns seemed in distress last night (at the top of the tank, heads out of water) and this morning they were dead.
Checked water prams 0 ammonia, o nitrite,0 nitrate and 7.8 ph(i know that is a bit low).
I have a 6 line wrasse and he is fine this morning. I also have a couple snails, 2 pepermint shrimp and a blue hermit crab and they are all fine.
There is an air stone for bubbles in the tank as well. I read on another post that air stones only work for fresh water but my lfs guy told me to use this.
Any ideas why the clowns died? What did I do wrong?

 

That is a tough one. Not knowing how much slime you had, my only thought was that the slime released a toxin. I have read about that however it is un-usual for that to have happened. I will inform you that increasing flow will in no way resolve nor abate the Red slime. That is a myth. I have seen Red slime growing in full force flow and it was the heaviest in the flow itself. I wouldn't blame yourself. There are occasions when the use of "Red Slime Remover" or "Chemi-Clean" are warranted. As long as it is not an out of control mess, reduced feeding and filtering techniques may just clear up the situation. If it were a toxin though, seems odd that just the clowns were affected. Troubling to say the least. I will tell you from experience of using the products I mentioned, they have never had a negative effect on my tanks. I would say they are completely reef safe and I would still use them with confidence.