I just setup my CO2 system last weekend and it is time for me to perform a 50% water change on my 55g tank.
My pH has dropped from 7.6 to 6.6 and I am waiting on getting a drop checker mailed to me to properly test my KH levels.

I've been working with fish for many years now, and I wanted to ask this to be safe.

When dosing ferts while following the EI, what is the best method to change the water in your tank?

My tap water is 7.6 pH and I have Cardinal Tetras, Rummynose Tetras, Hatchetfish, Clown and Khuli Loaches, as well as a couple exotic plecos in my aquarium. I am eventually going to add a couple Discus too (I just lost one after an accident while I was away at work).

Any input is greatly appreciated as I am worried about shocking the fish when changing a full 50% of the water!

Thank you,

I can only speak for the cardinals and rummynoses ... they will not mind a 50% water change from your tap. My pH for both my tank and tap water are an exact match of yours.

I've heard that Discus are very sensitive to pH changes, so I think you would have to do the water change differently for them.

I have discus and do 50% water changes twice a week. No problem. All I add is prime to the water. Only use CO2 to reduce pH. I run CO2 1 hour before lights turn on to 1 hour before lights turn off.

I have done even larger changes, fish do not mind..........pH is not the change you need to concern your self with.............every ADA tank and perhaps 70% of any CO2 enriched planted tank goes through a pH change every day of at least 1 full pH unit or so..............

This is not due in any way to TDS..........or salts, ...............or due to osmotic pressure...........CO2 is not a salt or adds to TDS..................

And since we add CO2 gas and modify the pH, the advice based on pH change does not apply, the pH advice is centered around large changes due to poor water quality and TDS only...............

Bottom line, do not worry about pH, worry about adding too little or too much CO2 gas.

Regards,
Tom Barr

I have a 90 gallon discus tank with two power filters. I'm thinking on getting a new filtration system. What do you suggest? I already have a wet/dry filter ( someone gave it to me as a gift) that I'm not using but it doesn't have a pump.
Should I get a new canister filer? (I'm using CO2) or buy a pump and use the wet/dry? If it is a canister, which one? how much gph should i get? thanks !!

Thank you all so much for getting back to me. I'll be doing a good 50% water change tomorrow since I ended up adding 10 Otto cats into the tank tonight and don't want to stress them out too much.

If I could delve into this a bit further...
A fellow hobbyist of mine sent me a message on another forum with a response. He told me that a single point pH drop represents only a 10x difference in concentration, where as a 2 point pH drop represents 100x, 3 point - 1000x. I understand the underlying message that a single point drop has a very low difference in terms of concentration, but I was wondering if someone could elaborate a bit further for me?

Thanks again,

p2bc,

Your friend is correct... Here's a link that you might find interesting:

pH - Wikipedia, the free encyclopedia

As far as my aquarium goes, I've been listening to Tom's advice of not worrying about pH for quite a while now (haven't checked it in months) -- my fish are doing fine.

I had troubles doing the 50% water changes with EI. I would usually lose a fish within a day of doing the water change, for no apparent reason. When I reduced my dosing and my water changes to 25%, things got better. My tap water has 0 GH and KH out of the tap, but my tank has sand in it that would increase KH, plus I would add GH to the water. I think the 50% water changes were causing the KH and GH to fluctuate too much, causing the stress to the fish. In most people's normal tap water, this shouldn't be a problem, most people don't have 0 KH and GH. Also, it probably would have worked ok had I been very careful to reconstitute the water added in appropriately to match the tank water, but I was not being very careful.

If there is concern, do a 25% water chaneg and observe. Then do another a few hours later. If all works out well you are probably safe to stick with a 50% the next week.

Changes in the KH will influence things, changes in the CO2 gas content will not(within reason-based on toxic levels in combination with O2- respiration is a two part process=> CO2 out, O2 in).

Once you realize this, you can see why the CO2 gas has little toxic erffects on fish due to pH change.

Do not think in terms of pH, think in terms of CO2.
That is what you add, that is what will kill fish potentially, that is what we add for the plants(plants do not care about pH particularly in aquatic systems, they care about CO2)

We are not changing anything else, we are just adding CO2 gas to help the plants, pH drop is just a consequence of that, pH can change for 101 reasons, that may or may not be any concern or do any harm to fish.

CO2 ppm change is very specific on the other hand..........

Regards,
Tom Barr