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Discussion Starter · #1 · (Edited)
Hi Everyone,

Thanks in advance for your advice. I did a water change on my tank yesterday during the day and woke up this morning to a tank with a lot of dead fish, and the rest gasping at the surface. I've since turned the lights on to get the plants to produce O2 and also added a bubble maker (which isn't always on, but I keep in reserve if needed) and the remaining fish are now breathing normally. Given that the fish were gasping at the surface and the problem has been fixed by taking the aforementioned steps I am pretty sure that this had to do with some sort of oxygen shortage. Since something similar happened about 6 months ago following a water change (also overnight when the lights were off) I am certain that this event occuring with a water change is not a coincidence. Can you advise on why this is happening?

Before I go on, some facts:
- 90G established tank full of 2 year old rainbowfish.
- No, I do not inject CO2
- My tank is a low tech tank that is heavily planted -- lots of moss, vals, java ferns, crypts, and more moss
- I run dual Fluval 406 canister filters. Filtration is fully cycled, and I have not cleaned either canister so recently that this would be the cause.
- I do a water change once per week, usually about 30% but yesterday was probably more like 40%
- When I do a W/C I tediously fill buckets from a bath tub to replace water -- I do this because I add Seachem's dechlorinator (Prime) directly to each bucket before pouring back into the tank. I also make sure the temperature is about even with what's already in the tank.
- As of last night when the tank lights were still on, all the fish were fine. I know because I fed them while "distracting" my infant son with their bright colors... there was no gasping at the time (I have learned to look for this after a w/c).

So, again, seems like something correlated with night time (plants respiring CO2?) following a water change. Can someone explain why this is? And can someone explain why this doesn't happen after every water change (which I do weekly)? I've heard about this in tanks with CO2, but not otherwise.

Thanks!
 

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I had this same thing happen to me a few times last year after moving in to a new house. Exactly the same, it seems. I would change the water during the day/early evening and by morning I would find dead fish and others gasping at the surface. It took me a few weeks to associate this with a water change and one day when I was late for work I asked her to hang an air pump and let it run. That seemed to resolve the problem. So the next week when I did a water change I turned the pump on just for that evening and things were better. Now I run the pump 24/7 and have had no issues. Another user on here, Diana (I think), suggested the problem could have been dissolved gases in the tap and suggested I aerate the water prior to adding it or let it age over night. This could be something you could try if you have the forethought and space.

Do you add Glut or Seachem Excel by any chance? I read that could decrease the amount of O2 available to the fishh, but since so many others do it without problem I doubt that would be the issue here unless it was an overdose.

You could also try contacting the water company and see if they've started doing anything different. There might be a notice somewhere on their website?

Ultimately I never did figure out the cause because experimenting usually resulted in more deaths. Now I just run a small air pump 24/7 and all has been fine. It's not my favorite thing to do, but it has prevented mass die offs on water change days. Good luck in figuring this out!
 

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Well, we can start with the water change. If this has happened twice in conjunction with a water change, and not at other times, then something about the water change did this. Here are some ideas, but who knows what the real answer is? If any of these are part of the answer, perhaps it is a combination of things.
As psych points out, find a solution, even if the problem is not fully figured out. Running a bubbler for overnight sounds like a good idea. Put it on a timer so it comes on when the lights are off.

A) Water in pipes can hold more gas (actually air, including nitrogen, oxygen, CO2 etc) or it can hold less. Without contact with the air it cannot gain or lose gases. Suggestion: Gas bubble disease shows up immediately (bubbles on tank walls, plants and on fish). But lack of oxygen (all the gasses, actually, but O2 is the important one) may not show up right away. Large water change followed by lights out (plants use O2) could result in reduced O2 to the point that the fish could die. This seems extreme, especially if the tank was refilled by bucket brigade. The act of filling the buckets usually provides enough air-water contact to add the gas (again, this is air) back to the water.

B) Extreme overdose of certain additives, including dechlor, can lock up the oxygen. Dose up to double of most products (especially if the water change is 50% or so) is completely safe. More than double dose may be a bit chancy if there is also something else going on.

C) Filters slowing down, such as if they have not been cleaned in a while. Combine this with refilling the tank higher than it was before the water change. Slower filters usually do not jet the water out across the surface, but sort of dribble it into the tank. (or somewhere in between) so the surface agitation is not like it usually is. Slightly reduced arc of returning water was OK when the water level was dropping a bit (such as right before a water change- who tops off then? I just say... Water change tomorrow, I will refill it then). But combined with a minor lack of oxygen perhaps from a combination of things, and this could contribute. This would also explain why it does not happen with every water change- you probably clean one or the other filter often enough that the cleaned filter provides better circulation for 2-3 water changes. Rare that both filters have not been cleaned for long enough to cause this.

D) For whatever reason, plants are growing better (so use more O2 at night), or more fish food so microorganisms demand more oxygen, or snail population increase.... SOMETHING is using more oxygen. Leaving less for the fish.

E) Warm water holds less oxygen. If this is a very warm tank (upper 70s or low 80s) perhaps you are adding a high % of hot water, so your bucket of water simply cannot hold very much oxygen. So even though you are splashing it and stirring it as you fill and treat each bucket, it is simply not capable of holding more oxygen. Similarly, the tank also does not hold much reserve of oxygen, but depends on good circulation (see point C) to maintain good O2 levels.
 

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Don't parts of NJ have somewhat soft, acid tannin rich waters?

Speaking from someone who suffered a big fish loss from a water change on a municipal water source. It your city's water district treats for tannin, ( like ours does ) they will sometimes do semi annual flushes of the big water pipes to clear out some of the floculants used to precipitate out the tannin in the treated water, which can cause cloudiness in home water. These flushes are sometimes accompanied by some changes in levels of Chlorine/Amines and other changes in the water that can wipe a tank of fish out. It's common knowledge in our local fish keepers group, about the city water, and I'm not sure if our Water District will warn people ahead of time, they used to.
 

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That is a good point, GrampsGrunge-
Water company can indeed treat the water differently so that whatever worked in the past is not correct for perhaps just one water change, or for the season, or for the rest of your time in this house (on this water system).

Flush with extra chlorine or chloramine, usually a one-time flush, perhaps annually or linked to when they do pipe repair.

Change water sources, often seasonal, lasting several months. When they get water from different sources, or if some of these sources change depending on the rain or snow and other factors.

Or a total change in the way they treat the water, such as switching from chlorine to chloramine.

These are reasons to test the water with every water change. If you know what your water company does, then a dip-stick test for chlorine/chloramine might be needed, or a 6-way test (NO2, NO3, GH, KH, pH and Chlorine) might be really helpful.
I buy these in the 100 pack.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=23793
 

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Discussion Starter · #6 ·
Thank you, everyone. It turns out the water company is doing some sort of change around this time. Lesson learned to test the tap water (not just the tank water) before changes in the future. The one thing I still don't understand is why the problem doesn't really occur until the lights are out... I changed the water around lunch time yesterday and the fish were fine all day.

I'm also going to run my spare pumps for 24 hours after a change from here on out as a best practice. I find them to be too annoying to run permanently, because of the sound and they mess up my plants.

Really, really frustrating... I lost 6 nearly full grown rainbows today, and not the kind that are easy to come by (Kamaka, Axelrodi)
 
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