Fish acting strange then dying

[xiiutao]

Hello, I have a 75 gallon fish tank with 3 fish in it. 2 gobies and 1 orante bichir. My father has a 42 bow front tank with 4 fish in it a small gar, senegalis bichir, parrot fish, and a scat. About a month ago I was away with my GF and my father volenteered to do my water change for me. Which was fine because we have both kept fish for about 2 years. Well, I get back from my trip and that week one of my gobies is having a problem. He would swim eratically then seize of like he had died. Gills with popped out and turned upside down. So I moved that fish into a 10 gallon tank I had running. That didn't help the fish would still do the same thing. It would come randomly, and happen once an hour to 2 times an hour to never in a day. Well, about a week later the fish finally died. Right after that death my other goby started the same thing. Meanwhile while that goby was in the 10 gallon one of my fathers scat starts acting the same way. Both of those fish die. Nothing happens within the next month and a half until today. My fathers gar decides to start doing the same thing. We are both clueless, and our LFS in the area don't help much.

My water changes consist of 50% water a week, with water conditioner.

My fathers water changes consist of 35% water a week, with water conditioner, cycle, salt, and algae destroyer.

He has used all of these products minus the algae destroyer weekly. He uses algae destroyer when he has algae. I don't use these products because I read they are not needed and can cause problems. I know for a fact he used these products in my tank when he did my water change a month ago.

any ideas or suggestions will be helpful. thanks

 

[aquariumguy2300]

Possibly the pH, or the water conditioner. I accidentally added too much water conditioner once, and one of my fish acted exactly like you described. She was swiming sort of sideways, then she turned completely upsidedown. I gently nudged her to see if she was still alive, and she swam upright again. She died a day later.It may also be the water change. Do you change 50% all at once, or scattered across the week?

 

[Nick660]

As it's happening to fish in 2 seperate tanks it sounds like it must be a common factor to both.
My guess -
Chemical additives used with water changes?
Something wrong with your tap water?

It's worth checking water parameters even though you're changing a fair amount of water regularly. If tap water params vary widely from your tank then this might also be a factor.