I have an undersink RO system (iSpring RCC7AK, NSF Certified 75 GPD, Alkaline 6-Stage Reverse Osmosis System, pH+ Remineralization RO Water Filter System Under Sink, Superb Taste Drinking Water Filter https://a.co/d/irg37YY) that I’ve been using for about 2.5 years.

About half a year ago, I started noticing that the Total Dissolved Solids (TDS) of the filtered water slowly rises as the filtered water sits in the storage tank (3 gal holding capacity). I did some research but couldn't find anything similar to my situation. I iterated with the customer service for like a couple months. They were super responsive and friendly but didn’t really solve the issue at the end.

Here’s the situation.

  • Incoming water TDS 600-700 ppm
  • After RO filter TDS 40-50 ppm
  • After alkaline filter 50-60 ppm

Above numbers are approximate and were taken with the freshly filtered water. I get these numbers consistently.

After the filtered water sat in the tank for about a day (=after a day or little or no usage), the water that I get from the faucet (after alkaline filter) becomes like 70-80 ppm. After another day, it gets to about 90-100 ppm.

I took all TDS measurements after I let the water run for about 30 sec - 1 min because the stagnant water in the alkaline filter compartment makes the reading temporarily high right after the faucet is turned on.

Here’s what I’ve tried so far. But none solved the issue.

Other than discarding stored water daily, I couldn’t find anything that would keep low TDS for the water in the tank.

The incoming water is from a well. I had my well chlorinated and system flushed about 3 years ago (a couple months before I installed the RO system). I tested the water for bacteria growth and it came back negative (nothing fancy, I used an affordable DIY kit from amazon). I also have a water softener and iron filter for the incoming well water.

How can I make this filtration system work properly, without allowing dissolved solids to re-accumulate in the storage tank over time?

  • Symptoms would appear to suggest that the storage tank itself is the problem. I guess you could try decanting to a clean glass jug and let that sit for a day to compare.
    – Ecnerwal
    Commented Apr 3 at 14:40
  • The alkaline filter is adding dissolved solids to the water by design. It may be that it is also adding a little undissolved solids (i.e. fine particulate) which dissolve as they sit in the tank, slightly increasing the TDS.
    – Mark
    Commented Apr 4 at 1:19
  • The alkaline filter is after the tank. Is it possible that with the faucet closed, there's some backflow?
    – jealer
    Commented Apr 4 at 2:03

1 Answer 1


To narrow down possible causes, I think you should temporarily eliminate the alkaline filter to see if the TDS is rising in the tank or after the tank. I realizing that you're flushing the alkaline filter for a minute, but that may not be enough to completely eliminate the TDS rise attributable to the alkaline filter.

If you find that the TDS is rising solely due to the alkaline filter, not in the tank, no worries. If it tastes good, it's good. Natural mineral water gives a much higher reading. Just do future testing before the alkaline filter. A T with a valve would make that easy.

The comment above about leaving RO water sit for a day: If exposed to room air in an open container, the water will absorb CO2 from the air, the pH will drop and TDS will rise.

Also, I've used both Culligan and Express Water RO systems, and I've always got MUCH lower TDS than you're getting. My softened well water pre-RO reads 350 - 500, depending. After RO it's 4ppm. No alkaline filter, just carbon. A new membrane starts about 10-11ppm, drops to 4 in a week or two and stays there for many months. I'm currently on month 32 on the same filter at 4ppm. When it gets up to 7ppm, I'll change it.

So 40-50 ppm seems high.

  • Jugs have caps/lids. Use that, no absorbing anything from the air.
    – Ecnerwal
    Commented Apr 4 at 16:18

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