The DI is supposed to remove the TDS completely from the water. Usually, the DI filter is used after an RO membrane for lengthening the life of the DI. Even if the RO membrane is reducing the TDS, you do not know what's in that remaining TDS. If it's nitrates, phosphates, silicates, etc., it could be that it's exhausting the DI faster than normal. If it's not that, check and see if you are allowing the water to flow too fast through the DI. Normal operating flow through the DI should be between .2 and .5 gallons per minute. Any fast than that and the DI will either exhaust faster or not reduce the TDS down to zero (though it will still be low). It could also mean that you need a special mix of DI resin other then the 50/50 we use as our mixed bed resin. A quick and easy test to determine if your DI is exhausting too quickly is calculated as follows. For example: If your tap water TDS is 200 PPM your water after the membrane should be around 10 PPM. Divide 3000 by this 10 PPM and it equals about 300 gallons of RODI water you will make before your DI cartridge is exhausted. This is for a 10" refillable cartridge. The In-Line DI cartridge will only last half as long. The 20" refillables will last twice as long. Yes, you do need a TDS meter! Please contact us if everything checks out, but the DI is still exhausting too fast.