Water change with RO water killed my Ghost Catfish

[scotsman1000]

OK so I messed up today with my 36g tank. It's about a month and half old and I introduced some glass catfish a week ago. I came to do my weekly 50% water change with RO water (been doing 50% as the tree roots I have in the water are producing quite a lot of tannins and turning the water brown-ish)

My system so far has been to:
1) Use a vacuum to drain the tank to 50%, vacuuming the substrate as I go.
2) Pour in RO water
3) Re-mineralize with 5 squirts of Elos Rigenera
https://www.elosamerica.com/store/#!/6-Rigenera/p/81644429/category=22919321

I did this but annoyingly I didn't have quite enough RO water on hand to complete the job and had to go out for an appointment, so left the tank for 2 hours. It was filled to 95%, no filter running and no re-mineralizing introduced .

When I got back the fish were clearly stressed. Lost 3 out of four glass catfish, one cherry shrimp (perhaps more), and one neon tetra looking very sad. Also the tadpoles that my kids put in were dead.

So, I am assuming a sudden swing in GH was probably what did it, and I know glass catfish are sensitive.

My questions are:
1) Is it better to re-mineralize the RO water before introducing it back into the tank?
2) I am a little confused about the Elos product. Better to use something like a Salty Shrimp Mineral GH/KH+ where I can be more precise with the re-mineralisation process? Elos's system is built around 30% water changes, and I have been doing 50...

Current tank parameters are:
pH: 6.8
Ammonia: 0
Nitrite: 0
Nitrate: 5
KH: 2 dKH
GH: 1 dGH
Phosphate: 0

Tank occupants are:
Amano Shrimp
Cherry Shrimp
Carbon Rili Shrimp
Glass Catfish
Neon Tetras
Emerald Dwarf Rasboras

Looking into it now I do see my GH is pretty low. I think I should aim for 6-8 GH, 3-4 KH - particularly for the shrimps. Am I on the right direction here?

 

[Autumnsky]

Sorry that you had a hard experience there. You are correct down the list there on what went wrong.

Imho getting the water close to what it should be before putting in the tank would be preferred.

Were your numbers on the tank listed before or after the RO water?

 

[scotsman1000]

Thanks! Those were the parameters after the water change.

I just bought a large tub so I can do the remineralization before adding. Going to try today.

Wondering what caused the most stress?
- different GH
- RO water being poured quickly in
- tank sitting without filter running for 2 hours

Combination of above?

 

[trennamw]

I haven't used RO water, but I live in an area where the tap water is extremely soft. I add products to bring the PH and GH up. If I'm going to pour it in, I test the tank and the "new" water to be very very sure they're the same. Otherwise, I set a bucket of re-mixed water up high and use aquarium tubing to very slowly add the new water to the tank.

After a year or so without any incidents, I started to deviate from this. But at first I took pains to keep it as gentle a transition a possible.

I could be completely mistaken, but can't tannins be cleared by running extra charcoal in the filter and changing it often? That may affect the tank in other ways, of course.