Sudden fish deaths

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Hi everyone. I'm new to the forum but I could really use some help. Many of my fish were found dead this morning. A big clown, a dragon net, and a six line wrasse. One shrimp did die yesterday but I checked all my parameters and nothing seems to be off except the ph level. So I don't think the shrimp death is what caused the fish to die. My current parameters are:
Nitrates 0
Ammonia 0
Ph 7.7
The tank has been stable for a long time. None of these fish were new I have had all of them for many months, some even years. I have no idea why they suddenly died but if anyone has any ideas I would love to hear them.


Staff member
pH may be too low for the fish you have. Good general pH for SW fish is 8.0-.02. What is your salinity level and what are you using to measure salinity? What about temp? Recent cleaning in tank? Malfunctioning equipment? What fish is left in the tank and how is he doing? Aggression? Nitrite level? Size of your tank?
Yeah I thought that ph might be the problem. Do you have any ideas on how to raise the ph. The salinity is 33pp and I use a refractormeter to measure it. The temp is in a good range. I did clean the tank a couple days ago. That can sometimes casue things to die but it isn't usually this many things. The equipment seems to be working fine. I still have a damsel fish, a small clown, a goby, and a sand sifter. I didn't see an aggressive happening. I tested the nitrate levels and they were fine. The tank is 70 gallons.


Staff member
What is the nitrite level?

Do you sand vacuum? Disturbing sand can sometimes result in releasing hydrogen sulfide from the bed--potentially deadly.

What kind of water circulation do you have going in your aquaria? Throughout, such as surface, middle, bottom?

Take a look at this article on pH. It's a long read, but it should give you the info you need to assess your problem and work at resolving it. Not usual for a saltwater tank to have that low pH. Normally, the salt itself will keep pH at least at 8. You should be able to find a product in your local fish shop to help raise pH. The goal is to raise it very slowly, small increments.
I do move the sand around sometimes when I clean. That could be the problem. The water circulation is at the top of the tank. I believe the nitrite levels are ok. I checked all the parameters this morning and the only thing that was off with the ph. Thanks for the help. I will check the article out and try to raise the ph.


Staff member
Check your nitrites. Also, a marine fish tank needs water circulation going throughout the tank. You can easily accomplish this by adding powerheads in the tank to ensure that you have water move evenly distributed throughout the tank. That may be a contributing factor for your pH problem.

What salt do you use? What is your water source?
My water source is the well water at my house. I have checked for Cooper in the water just in casue the pipes are putting Cooper into the water but there is never any. Should I add powerheads to the bottom of the tank? I use aqua forest reef salt. My nitrites are at 0 ppm.


Staff member
What kind of system do you have set up where you can attain zero nitrates? Is it a reef?

Yes, add enough powerheads so that you can get circulation throughout the tank. What is moving the tank water at the surface?


Staff member
Beth she said nitrites at zero, not nitrates. Could be a typo tho. In almost any tank there will be nitrates
Stirring up the sand is a big no no tho. All kinds of toxic stuff gets trapped in there


Staff member
P.H. of 7.7 shouldn't be lethal by any means unless there was an enormous swing. However, if you've disturbed a significant portion of the sand bed and or rockwork prior to the losses then that's most likely what killed the fish.

Bacteria in sand and rock produce hydrogen sulfide as a byproduct of converting organics into energy. High levels of it can build up in anoxic zones, i.e. sand beds, under rocks or other areas that get little to no flow. It can easily kill all of your fish and inverts if enough of it was released in a short amount of time.
Yeah my nitrates and nitrites are both at zero. I haven't really had a problem with them. It's a well established reef. Thanks so much for the tip about the sand. I wouldn't make that mistake again. I'm glad the ph isn't lethal right now, but I do want to get it back up to the right level.