Subtropical 75 gallon planted tank - Why are my fish dying?

[AquaAlic]

I have a subtropical 75 gallon planted tank, lightly stocked with less than a dozen each of zebra danios and neon tetras as well as about half a dozen bronze cory cats. Over the past week I've lost 4 danios and 1 cory cat, about 1 fish per day. I see some danios seem to be "panting"/struggling to get oxygen, not their normal active selves. There is also a single platy, clown pleco, and mystery snail in the tank, all of whom seem fine. The neon tetras seem fine, just the zebra danios & bronze cory cats seem off.

I have maintained 0 ammonia and 0 nitrites, the nitrates were a bit high around 40 so I did 30% water changes ever other day until they got down to 5. Maybe nitrate poisoning? But seems unlikely given the stock of "bulletproof" fish. Been keeping aquariums for a decade, recently broke down all tanks and combined the "easy" fish into 1 big tank to make life simpler since we have a toddler and baby. Last week our A/C went out, I'm thinking with normal water parameters maybe this was the culprit? I know the 2 species that appear to be suffering like cooler temps, the water got up to 82 when the A/C was out. It's back in the upper 70s now and as fall approaches will continue to drop back into a more ideal range in the mid-lower 70s. Could this just be the after effects of the heat wave, or does it seem I might have introduced a disease? I've been dosing with Excel to treat a little black beard algae on the sword plants, could this reduce the oxygen too much? I use 5 mL per day with a turkey baster directly onto the sword plant leaves that are affected. If that's hurting the fish I'll happily just cut off the infected leaves instead.

I eventually want to get a much bigger school of neons and larger group of corys, but obviously need to figure out and get the current situation under control first.

 

[mattgirl]

As the water heats up there is less oxygen in it. Unless you already have 2 or three I recommend adding extra airstones or sponge filters. Anything to get more oxygen in the water. You are correct, danio prefer cooler water and the corys spend the majority of their time at the bottom of the tank where the least amount of oxygen is going to be.I can see warmer water affecting danios and lack of oxygen affecting corys.

 

[AquaAlic]

I just have a large canister filter on the tank, I do keep emergency battery air pumps around in case I lose power, I can try sticking some in the tank and see if that helps. If that works I don't mind buying one to use in summer months if that would help keep everyone happy. I've been debating if that or a powerhead might help?

 

[Mhamilton0911]

a powerhead might help

If you think you'd benefit from the extra flow from a power head, some models include an air port for adding extra oxygen into the water

Quick amazon example

 

[AquaAlic]

Hey thanks, looks like I could even add a small sponge on the intake of that kind of device for an extra little bio filtering?

 

[A201]

In days gone by, Zebra Danios were a lot more durable. Zebras now are often the subject of various disease related on line posts & threads. "Wasting disease" seems to be the most common complaint.
Healthy Neon Tetras are a pretty good indicator of water quality. The fact there have been no losses suggests a very good environment.
Generally speaking, Corydoras are a relatively fragile species. Occasionally losing one out of a group is troubling, but not unusual & doesn't necessarily indicate a disease outbreak or a water parameter spike.
Adding aeration is beneficial to most setups & I hope it helps your situation. IMO, the noted fish die off was most likely due to a weak batch of Danios.

 

[mattgirl]

I just have a large canister filter on the tank, I do keep emergency battery air pumps around in case I lose power, I can try sticking some in the tank and see if that helps. If that works I don't mind buying one to use in summer months if that would help keep everyone happy. I've been debating if that or a powerhead might help?

Since I've not used one I am not sure how a power head works. Personally I don't think you can go wrong in adding a sponge filter or two to your tank. It adds aeration and is a great place for bacteria to grow. Once well seeded it can be used to quickly cycle another tank should you need one in an emergency. I actually run 2 along with 2 HOB filters on my 55 gallon tank.

I agree with A201 Danios aren't nearly as tough as they once were. I have kept them for years. Unfortunately the last ones I got ended up with a bent spine within a year of getting them. I lost them one by one until all 6 were gone. I chose not to replace them.

Warmer water =less oxygen so either way, power head or sponge filter, our tanks need the extra oxygen bubbles breaking the surface of the water provides when the water heats up.

 

[AquaAlic]

Thanks to everyone - for now I've MacGyvered a battery air pump and an extra tetra internal 40 filter for water circulation & aeration and within an hour the fish are acting much more "normal". I incorrectly assumed since I got a big canister filter that would be sufficient for the tank!

Now to decide between air pump or power head for the long term solution...

 

[mattgirl]

Thanks to everyone - for now I've MacGyvered a battery air pump and an extra tetra internal 40 filter for water circulation & aeration and within an hour the fish are acting much more "normal". I incorrectly assumed since I got a big canister filter that would be sufficient for the tank!

Now to decide between air pump or power head for the long term solution...

You can never go wrong with more filtration than is actually needed but as you have found, water movement needs to be taken into account too. 'Specially breaking the surface of the water during the warmer parts of the year.