What did I do wrong?

[ADMWNDSR83]

On Sunday, my multies were looking well, but my nitrates were a touch high.  I decided to do a water change, and switched out about 8 gallons of water, using Prime for chlorine and such.  This morning, two of adult multies were dead, and the other two adults were not looking well.  I imagine I'll find them when I get home from work today.  There were 8 fry in the tank, and this morning I could only see one, swimming uncharacteristically high in the water.  There are also quite a few trumpet snails in the tank, which usually stay buried for the most part with one or two at any given point in time on the glass.  This morning, it seemed all of them were on the glass, many up by the water line.   The only thing in the tank that seemed unfazed was my dwarf pleco, who actually seemed more active than usual, probably because he didn't have the multies sniping at him.

I did a parameter check, and everything seemed the same except the pH, which was a bit lower than it has been.  My water is naturally acidic, so I use aragonite as my substrate to buffer.  I'm wondering if my water change was too much water, and made too much of a swing, or what I may have done.  Any thoughts on how, if I try again, I can avoid another mass-casualty event?  Or should I just give up on shellies and try something more suited to my rock-hard battery acid water?

[Guppy Guy]

How big is your tank? How many fish total are there? 

It sounds like the aragonite didn’t work fast enough to raise the ph before the multies felt it. I like seachem alkaline buffer, so just add that to your new water before adding it to the tank. Also make sure that the ph is the same, or you will have more problems.

Let’s see what @Guppysnail @Streetwise and @Colu have to say.

[ADMWNDSR83]

The tank is a 30 gallon.  There were 4 adult multies, possibly 8 fry, and one dwarf pleco.  The only other change that was made during the water change was removing a lava rock formation and putting in some artificial decorations. 

[Martin]

When you say the pH was "a little lower than it has been", can you offer specifics like before and after?

[Colu]

When all the snails leave the substrate hang just below the surface it usually a water quality  problem low levels of desolved oxygen high levels of ammonia nitrites nitrates  you said your  nitrates high what where they and what was your pH 

[ADMWNDSR83]

Before Water Change: No ammonia, no nitrites, nitrates about 10-20, pH about 8.2-8.4

After water change: No ammonia, no nitrites, nitrates nearly white, pH about 7.8

[Guppysnail]

I have to test my tap water every single time before decolonization. It goes from 2 ppm to 4 ppm. Dechlorinators on average treat 1 ppm chlorine from my loose chemistry understanding. So I test to make certain how much to use. Also what size tank?  Doing 8 gal in a 20 is different than in a hundred. If it’s a smaller tank and was lower than normal and your tap is significantly higher it may have given to a drastic ph possibly?  I’m sorry for your loss. 

[ADMWNDSR83]

Tank is a 30 gallon, possibly 37.  It was given to me a few months ago, and I was told 30, but it looks the same size as my 37.  I know it's the same lid.

[ADMWNDSR83]

Just for some follow-up, I just got home from work, and it looks like the two remaining adults are doing well.  I haven't fed them yet, but two multies and the pleco are all active.  I don't see any of the fry, but I may be missing them, so we'll see.  It does seem like a male and female remain so who knows what's coming!  I will probably look to get four more once I'm sure everything is ok.

 

I will note the evening test shows the pH back up above 8.4.