[GringoPhil]

A lot depends on what you are going to 'dispense' in your tank.
I hope you are not thinking of using one of the formaldehyde based chemicals. The main thing is to have good aerobic bacteria out number the bad bacteria, and you should not have any odor. A bit of Mean Green, Pure Power, Roebic, K.O, Ridex, or the best, Eco-Save will keep you odorless, without chemicals.

< Active BACTERIA will do the job. A properly set up and maintained system won't need much either. As opposed to chemicals (where more and more must be used with less and less effect), when adding bacteria "less is better." It's important that you follow the instructions. You must start with a clean tank (or residual chemicals will defeat the action). You need to start the tank with a few gallons of water. You shouldn't add the bacteria until there's a deposit of human waste in there. If you do it right, it will take about three, in normal use, dumps before you have a good, renewable system. You can then keep it going with only small additions of bacteria every other dump or even fewer.>