If your doing a DIY CO2, the answer is basically nothing. As long as your GH and KH are adequate (higher than 3degrees, preferably 5), and your tap pH isn't in the 6's to start (won't be if your hardness levels are fine), you can do no wrong with CO2. Try not to fluctuate the CO2 too much (don't unhook it at night, or make 2 batches and blast the CO2 in the tank too quickly), as this could stress the fish or invite algae, but CO2 really is harmless. Many people devise ways of trapping the CO2 in the tank as long as possible before the bubble reaches the surface. Some make reactors with powerheads, some have spiral "staircases" for the bubbles to slowly move upwards, some just stick the tube in the tank and hope for the best.
Personally I have a terra cotta pot upside down in the tank and have the tubing going into the pot. The pot is on a slight angle and takes about 100 bubbles before a giant bubble rises to the surface. This has worked extremely well (actually too well due to my small tank) and is probably the cheapest, easiest way to make sure the CO2 has good contact with the water column. If you have a HOB filter (I have an aquaclear) try to get the water level as high as safely possible in the tank so you don't agitate the CO2 out of the water. I also set the power setting to the far right during the day time (lowest flow) so the max amount of CO2 is in the water, and then at night set the power setting to the far left (max flow) so the least amount of CO2 is present and better filtration of ammonia and nitrIte occur. Remember if you have a good amount of plants there is no real reason (other than eliminating the ammonia and nitrIte) to have the filter flow be maxed out since the plants are producing a large amount of oxygen (during lighted periods). If you have a canister filter you really don't have a problem since there is no waterfall effect.
Now high light, on the other hand, can be much more problematic if you just slap on a 1Megawatt lamp and hope for the best....
EDIT:
nitrous, the Hagen system is not really recommended on this site unless you have a really small tank (do you have a 20gallon?). It does not produce enough CO2 to maintain proper levels in larger tanks (I think anything over 20gallons). They are really REALLY easy to make and we can give you all the formulas for making the mixture. I know the Hagen is nice in that they give you premixed reagents, but it does not get that great reviews.