RO/DI water and pH crash

I'm using RO/DI in my tank now. my tap water was 8.6 and settled to 8.4 in the tank. The RO/DI water tested at 8.4 but I was told by a LFS employee that RO water doesn't hold pH and that it will crash in my tank. Is this true? And if it is does salt mix (I'm using Kent's) buffer the pH in the water or do I have to start buying buffer now also? This employee says that he goes through a container of Seachem buffer a week and still cant get his pH over 8.2! He has a 140g system I have a 40g system. I've done 1 small water change (10%) and started topping off with RO/DI water and my pH has already fallen to 8.2. I was going to do a 25% water change today but now I'm concerned that I'm going to crash my system.

 

I really do need an immediate answer to this post

 

Im no expert, but seeing how almost all of us use RO/DI water for mixing salt and top offs, if what he said was true there would be a lot of tanks crashing right now. The salt mix will buffer the water, low PH is usually caused by poor gas exchange.

 

I moved this to the ASAP board although I don't think you have to worry much here.
You should mention what salt you use and what time you did these pH tests as your pH will fluctuate in a 24 hour cycle.

 

I thought I did. I'm using Kent's marine salt. I mix it to 1.025 sg. I tested the water about an hour ago; 11am central time and got a reading of 8.2, down from 8.4 a week ago. Not sure what time that was taken but probably in the afternoon some time.

 

Check it again just before lights out, thats when it will be the highest. Ph drops at night unless you run a reverse light cycle in your fuge. It then slowly climbs back up in the day.

 

The pH will drop over night and rise during the day. If possible try measuring at the end of the day just after lights out. If they aren't taken at the same time of day then it's really hard to compare the values accurately. 8.2 isn't bad as long as the pH remains stable is the big thing.

Edit: got beat to it lol ^

 

Thanks - missed that - thought you were referring to a buffer.

 

yeah I don't know too many people not using RODI water! Good luck if you're not! Remember just because they work at an LFS doesn't mean they know what they are talking about or aren't just being a salesman!

 

I just back from a local fish store and asked what kind of water they used. He said tap. I said have u heard of RO/DI? He said yea, its not really as needed as you think, tap water introduces certain chemicals needed for the tank. I said ok and walked out...