Are you sure the new reading is correct as that sounds unlikely, and I would have thought you would have had shrimps dying with that sort of decrease. Do you have a second way of checking the TDS, or can you take a sample to a fish shop for them to test? I can't think of any reason that would happen or even heard of that before? Even if you had put pure RO water in last water change it wouldn't have dropped that far!
If it is currently the TDS113 it will make adjusting easy as you just mix the water to the desired end TDS figure (140ish) and it will slowly go up to that.
That's weird. TDS never goes down, it only ever goes up unless you did a water change with pure RO water, or perhaps a misreading from the TDS meter.
Try testing it again.
Check your TDS pen against Pure RO water. It should read close to zero.
Check the TDS meter against tap water, or something you kinda know what the reading should be to make sure that the TDS meter is still working as expected. To loose calibration by 100+ is quite a lot. You can try brushing the TDS meter's probe with a soft brush under tap water to make sure there is nothing effecting the reading.
I was going to do a water change this week and I checked my TDS and it's 113. Not sure how that happened. Last week my TDS was 240 after water change and my salty shrimp RO mix was 200. Maybe everything settled down? I'm at a loss. Anyone have any answers?