Yeast Explosion

     Yes. I had this happen once before, except the problem was because 
     some weird gelatin formed over the airstone and blocked the air flow.  
     The place where the airline was siliconed to the cap of the 2 liter 
     broke, and a fountain of foam covered my ceiling 6 feet above 
     it...just a little pressure buildup.
     I have a problem that I'm hoping someone can give me suggestions on.
     I've had all the fish in my tank killed by acidosis...twice now.  Both 
     times were as a result of massive pH changes due to my inaccurate CO2 
     injection method.
     The first time, the tank just ran out and the pH went from 6.2 to 7.8 
     in about two hours.
     The second time, last week the whole CO2 tank vented into the aquarium 
     at once--a one inch shell of ice on the CO2 canister was a bit 
     I have my CO2 canister, through a regulator, pumped into the input on 
     my pump.  The pump is connected to a 1/2" hosee which is connected to 
     a PVC pipe with sponges inside (hanging vertically).  The water is 
     forced vertically down into the vertically-oriented chamber with 
     sponges inside...it slows down the CO2's escape and traps some of the 
     bubbles in the sponge.  I think what happend was that the sponges got 
     clogged with mulm and during a short power outage, with the pump 
     stopped, a pressure release event like my yeast explosion 
     happened--the pressure built-up behind the plugged-sponges eventually 
     blew the intake off of the pump and vented the entire canister of CO2 
     into my tank...the pH was something lower than 5.5 (the lowest my kit 
     will measure) and all my expensive fish (including about 10 siamese 
     flying foxes which cost $8 + shipping ea.) were floating uncontentedly 
     It hasn't killed any of the plants yet, but the crypts are feeling the 
     strain.  I haven't had any CO2 in the tank for about a week now and I 
     really have to wonder what to do.
     I won't cope with another mass fish death due to massive pH 
     fluxuations and I don't think I can afford a fully-automated CO2 
     injection system--and I won't go back to my old unreliable yeast 
     method.  Isn't there a reasonably affordable reaction chamber 
     available to help dissolve CO2?  Even That Fish Place claimed to have 
     no such thing.  I had to makeshift--and it turned out to be the death 
     of my fish.
     I will no longer keep expensive fish because I can't afford to replace 
     them.  I have two 10 lb. CO2 tanks and a regulator--anything I can do 
     that will make a reliable system for $100 or less?
     Thanks in advance.