Check to make sure you have a prefilter (spooled string or some sort of polymaterial) then a carbon block, then the RO membrane, then a D.I. cartridge. 25 TDS from a source of 300 sounds like your R.O. is working fine, but as the others have said, maybe no D.I. cartridge, or the D.I. cartidge is exhausted. Most R.O membrane have a rejection rate of about 90%, which is why you have 300 going in and 25 coming out. If you do have a D.I. cartridge, try disconnecting the line to it from your R.O. membrane and check the TDs at that point. It will probably be around 25, or should be. If it's a lot higher, something's wrong with your R.O. membrane. To be honest, I wouldn't worry much about putting 25 TDS in your tank, unless you have amazingly delicate animals.

If the TDS is 25 coming directly off the R.O. membrane and you want to go to zero, get a new D.I. cartridge, or if you have no D.I., you can get a separate D.I. canister. I've had great service from Bryan at purelyH20.com, and he's a MAAST sponsor. BTW, I would highly suggest you get a refillable D.I. cartridge and a bag of resin refill. I went through a cartridge in under 200 gallons. Getting a refillable will save you money in the long run.