CO2 Questions

Hi, I have a 10 gallon tank which I am using as my "first" plant tank.  I am 
currently using a DIY yeast reactor (1 cup sugar, 2 tsp yeast) to provide the 
CO2 for my tank.  I realize that the amount of CO2 is probably impossible to 
control for this type of reactor.  I read all sorts of posts about how people 
can get good bubbles with their reactor and I wonder how they do it.  My CO2 
only seems to bubble for 4-5 days.  In the beginning, CO2 is coming out of 
the reactor manically.  In fact, I lowered the pH 1.5 - 2.0 points within an 
hour (something my plants love, evidenced by a TREMENDOUS increase in O2 
production), but something that my fish hate me for.  After 3 days, the 
bubbling dramatically decreases.  Am I using the wrong kind of yeast?  I'm 
using the Fleishmann's yeast.  I thought perhaps it was the fact that the 
yeast settle to the bottom of the 2 liter bottle, so I rigged up an 
Erylemeyer flask with a rubber stopper which contains a stir bar powered by a 
mechanical stirring device.  This has made the yeast last about 2 days 
longer.   Any comments?

Also, is there a very inexpensive way to construct a CO2-pH controller?  What 
do the ones from Dupla look like and how much do they cost?  I want to try to 
maintain the pH of the tank about 7, but find that this is extremely 
difficult to do.  My water has a KH of 4, which I can increase (due to the 
fact that I dilute tap water with a free supply of RO water).  The pH 
fluctuates wildly with CO2 fertilization and the fact that I have a pretty 
small tank.

On another note, a question about shrimp.  I just added some shrimp and 
otocinclus to my tank, (previously I only had one black molly for cycling).  
I'm wondering about what kind of load the invertebrates put on my tank.  The 
shrimp are basically feeder shrimp, which the guy at the store said eat 
algae.  I got 7 for a dollar and tossed them in.  The guy said that they get 
no bigger than 2 cm, but I am wondering if I put too many in my tank.  Any 

Thanks for any suggestions.

Yi-Meng Yen