Plecos dying

[Bullsnark]

pH 8.0 

hardness 300ppm

total alkalinity 720 pom

nitrate 0 ppm 

nitrite 0 ppm

temps 73 and 79

OK so I have two different tanks that my plecos died in the last 24 hours. Last Sunday, I found tiny white spots on the fins of one of my female beta, so I hit all 4 tanks with the med trio to put a stop to everything. The fish had all been in two tanks previously, so I knew the chance of contamination was high. This Sunday, I changed about 50% of the water in both my 20 and 10 gallon tank, and found the first of the dead pleco while gravel vaccing. He had not been doing great from the beginning, so it wasn’t a surprise. What was a surprise is that 4 more pleco died in those 2 tanks within 24 hours of the water change. The water was treated with prime, and sat for 45 minutes before being added back to the tank(s). All of the other beta and guppies are doing fine, but what would be causing this plecocide?  
I didn’t include pictures, but that’s because the fish are in a plastic bag frozen right now. 

[Keeg]

On 9/8/2021 at 12:35 AM, Nooby said:

@Bullsnark so you have two tank with a total of 4 plecos just to clarify and I don’t understand the “fish are in a plastic bag frozen” part

A lot of people (including myself) will take their deceased fish and freeze them before dealing with the body. This allows time for people to handle the remains such as burial or taxidermy. Kind of like a morgue but for fish. 

[Keeg]

On 9/7/2021 at 7:22 PM, Bullsnark said:

What was a surprise is that 4 more pleco died in those 2 tanks within 24 hours of the water change. The water was treated with prime, and sat for 45 minutes before being added back to the tank(s). All of the other beta and guppies are doing fine, but what would be causing this plecocide?  

it could have been internal parasites, with plecos being bottom feeders, it isnt unheard of them sucking them in. My best guess was the added stress of a large water change, coupled with some sort of illness was too much for them to handle. 

[Colu]

What's was your ammonia level and you left you water to sit for 45 minutes is possible the temperature droped when you added it back in it shocked them 

[Bullsnark]

I can’t imagine that ammonia would be an issue as one tank had three pregnant guppies and two pleco, and the other had 3 beta and 3 pleco. The guppy tank is over half full of plants for the fry to hide in once they are born, and the sorority tank has a bunch of plants that have been growing in there. 

[Bullsnark]

Ok, here are test strips on my 4 tanks. Granted it is 3 days after the plecos died. To be clear, all the other pleco in both the other tanks and the one left in the beta tank are doing fine as far as I can tell. It’s hard to tell on an albino bristlenose though. The picture of the bottle I had to use the panoramic view and cut my hand out afterwards; it’s a mess, but you can see the chart clear enough. Nitrates, nitrites, water hardness, total alkalinity, and pH 

[Bullsnark]

I guess I should’ve added that the top left test is the guppy tank. Bottom left is the beta tank. Top right is the 55 gallon. Bottom right is another 10 gallon

[Colu]

I think it more than likely it was something to do with your water change as they all died within 24hr  I can't be 100% sure  that the case it but nothing jumping out at me