gH Spiking need help

[Minoul]

Current levels of tank
5 gallon tank
Amm: 0
Nitrite: 0
Nitrate: 5ppm
gH: 171ppm
kH: below 17ppm
pH: 6.8
Temp 77F
My issue: recently got an RO system and changed out my water 1/3 is RO 2/3 is TAP. Thought RO would get rid of my issue that my gH reliably climbs to above 200ppm once added to tank. Tap gH is 143ppm.

1 day ago gH of my aquarium was 161ppm
Today gH is 171ppm
I havent changed any water in 3 days.
Here are the full contents of my tank

Contents: ~15 RCS, crypts, dwarf sag, flourish tabs, imagitarium black sand substrate, bamboo wood, sponge filter, HOB aqueon quietflow filter (media is matrix, cuprisorb, porous ceramics, and sponge), baffle intake using brown pantyhose, glass feeder dish, airstone, 2 suction cup clamps for tubing, one thermometer

yesterday I removed the air stone and the cuprisorb filter media in order to test them as the culprits. No change in gH in the test water for either.
2 weeks ago I did a 100% water change to get my gH down and the tap watr I added (before I had the RO system) was 143gH ppm at the start, and over the course of a week it rose to 200+ ppm.
I use the API gH test kit.
PLEASE HELP!!!!!

 

[Sgooosh]

I dont know much about ro but maybe add a fresh sriftwood?

 

[Colin_T]

It is something in the tank causing the GH to rise and has nothing to do with the R/O water.

Take samples of everything from the tank and put them in different containers of R/O water. Check the pH and GH before adding the items, then again 24 and 48 hours later.

It's probably the gravel or shells if you have shells.

 

[Lajos_Detari]

As Colin mentioned, check everything that you put in the tank.

Check your sand, filter medias in the HOB, your fish/shrimp food(especially if you feed heavily) to any fertilizer that you are using.
Some shrimp food may have high Calcium that will affect the GH if your tank is small.

How to Lower Aquarium GH in Water Tanks & Reduce General Hardness

Beginners aquarium chemistry: water hardness (GH) This guide covers ideal GH levels, how to test, adjust, and more!

fishlab.com

 

[seangee]

I have seen several reviews that suggest that imagitarium black sand raises both hardness and pH. That would be my first suspect.