Why did my fish die all at once?

I have an establish 37 gal. aquarium. A week ago I did a water change and recently added a new heater and a sicce circulation pump for my CO2 diffuser.

Today I check on my fish and I lost one Neon Tetra, one Corydora, one Zebra Danio and one Amano Shrimp. All the other fish seem to be fine besides a female dwarf gourami who I think is dying of DGIV.

I just tested the water and got the following:

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 5 ppm
pH: 8.0
Water Temp: 77

The Zebra Danio was getting skinnier and skinnier but was still swimming ok. The Corydora, Neon Tetra and Amano Shrimp had no visible issues. I don't understand what would've cause them to all die overnight.

Any help would be appreciated.

 

Do you run the CO2 at night?
sounds like the added circulation took too much CO2 into the water column, suffocated the fish.

 

The CO2 and Sicce Pump are set to go on about an hour before my lights turn on. So 5am for both, lights on at 6am. Then CO2 and pump off at 6pm and lights off at 7pm.

Currently I'm at about 1bubble per second.

 

I see, I don't know how to dose CO2 or if that is enough to suffocate, so I'm just guessing after your description of what changed recently, the addition of the pump.

 

I see, I don't know how to dose CO2 or if that is enough to suffocate, so I'm just guessing after your description of what changed recently, the addition of the pump.

Yeah, I didn't think 1 bubble a second was an issue. I know with CO2 overdose you can normally see fish gasping for air towards the top. They aren't doing that. I could always turn it down and see if that helps, but the plants are oxygenating very well too.

 

How are you measuring your co2 1 bubble per second sounds low so i doubt that had anything to do with it. How long have they been in the tank. Other then getting skinnier have you seen any other signs of worms or other diseases

 

The CO2 has never been an issue for the past 4 months.

Regarding disease, the only thing I've noticed is one of my smaller Danios started getting skinny and looked physically smaller. That's the one that is currently dead.

Another Danio looks like it is getting skinnier now. The tetras looked good and so did the Corydora and shrimp.

The only other issue I have is the gourami. I had my male DG die of what I think was DGIV. Basically developed sores and bumps, holes in the fins and I euthanized him. The female is starting to exhibit the same symptoms. Sore on the lip, two above the gills and on the right eye.