Massive fish death

[Ihaveafishor2]

I have several tanks connected to a singular, massive filtration system (CASCO 12K). On Tuesday, I started to see large loss and knew a problem was coming. Fish swimming to the bottom corner and grouping together. Random oxygen bubbles along some tanks' gravel and some of that gravel has turned from a blue to white (tested for chlorine but nothing). Some tanks have had sand put in as their substrate and may have been contaminated, may not have been (but the colors don't match up and my boss thinks that the case). Either way, I make sure to stir up the sand at least once a week for oxygen and preventative purposes (if it's already contaminated then it's already there and should have already killed everything at this point).

New fish came in, all died, I assumed due to cold in shipping, but this was different. Now 90% of what we've had has died. Any ideas?

I did some things that were regular maintenance and made sure the chlorine wasn't the problem. I can't smell much due to a broken nose, but I'm told it smells weird, but not like rotten eggs or chlorine. I also made sure to get some oxygen and water flow going better in all of the tanks. Idk why my boss hadn't already, but if he thinks it's chlorine, that'd only help I'd think.

Any ideas on what to try or look into? Of the few that survived, several have come down with Ich today as well. My boss just kept brushing me off so I'm now looking out for help as I'm out of ideas...

 

[Aiken Drum]

Do you work in a fish store or public aquarium or something?

 

[Ihaveafishor2]

Yeah, I just know they get pissy if I mention it so I was trying to be sorta bleh about it.