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TDS going up >25 ppm/day

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I have a fluval spec v I'm trying to keep caridina shrimp in, specifically blue bolts, however I'm concerned with the TDS readings I'm getting. Daily they are going up by apx 25ppm. There are no fish in the tank, but I've had an Amano, and variety of snails in there for the past few weeks. It is a newly planted tank and I'm running co2. 


Background-Tank has been set up for over 4 years, I was keeping some chili rasboras (and algae) until the last one passed about 6 weeks ago and I decided to give the tank a good scrub to remove all the algae and rinse the substrate (eco-complete). I of course saved the biomedia and sponges before rinsing the tank. I have both an rodi system and well water that I just recently got tested, rodi is 0ppm TDS, well 15ppm (mostly comprising of silicates). I have multiple successful reef tanks and quit a bit of experience (over 10 years) in the saltwater hobby, but unlike many others I didn't start with freshwater/planted tanks and I always struggle when I attempt them. I mention this because I am quit familiar with water chemistry and tank cycling and what not, but I feel I am missing something. 


I have been using rodi water remineralized with salty shrimp gh+ to 120ppm TDS eversince refilling the tank after cleaning, and been doing water changes with both 0 TDS water and slightly remineralized water to try and keep the tds down but the following day it is back up again. I've stopped water changes for a few days to see if it will balance out and it has gotten up past 250 TDS. I've added purigen, and matrix to try and get more of a bacteria colony going in hopes of adding a few small fish soon. I also just bought a new TDS meter but it's reading the same as the last one. I also have an auto top off system that replaces evaporation with rodi water (in hopes to prevent TDS creep). 


Can't for the life of me figure this out.


Tank Parameters 

Temp 70

Ph 6.4

Kh 3

Gh 5-6 (seems to rise also, but when changing water that corrects it) 

Nitrate undetectable 

Ammonia 0

Nitrite 0

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Sometimes the tank reset is the culprit. Lotsa dissolved organics which messes with TDS. Other, maybe the old Eco-Complete might have absorbed a lot of minerals that are releasing back in the water. Might suggest getting rid of most of the substrate, starting clean with a different neutral shrimp-based substrate (rounder, smaller, softer) or perhaps a buffer substrate.

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