Lost 3 fish in a week

[New Boy]

One of my rosy barbs has gone missing, one died and a dwarf gourami have died in the past three or four days and I don't know why.
Nitrite = 0, Ammonia = 0, ph stable at 7.8, nitrate = 40 (about same as tap water)...fish had ripped fins but they may have been dead in the tank for afew hours and when I worked at a lfs any fish that was dead for more then 5 minutes would get it's gins shredded.....no other body marks

....possible reasons.....

I recently added two orange chromides, one I've seen 'snap' at one of the barbs, however I'e never seen them chase (it's only if the barb was right in front of the chromide) - could they be embarking on a killing spree when lights are out?

I've recently upped my water change regularity from 25% every three weeks to 35% weekly, too much?

I've changed from Aquasafe to Haloex for dechlorinator - haloex seems to only need a tiny amount for 100 litre water change.....

and prob most likely when the chromides first arrived one developed what appeared to be white spot so I treated .......two 50% water changes a week apart later the spots reduced but didn't clear up..... but only the chromides had them and they seemed larger than white spot in the photos in books so I changed my diagnosis and added some anti-internal parasite treatment - and it all cleared up. A week later and 3 fish have died in 3 days...could I have somehow poisoned the tank??

Any advice appreciated.
Thanks
Andy

 

[Miss Dib Dabs]

35% sounds quite high for normal water changes. And two 50% changes a week apart may well have had an effect on the eco-system.
More likely is the new arrivals. They could be carrying parasites or fungal infections that aren't visible to the naked eye.
Why did you change to a different dechlor? Not that that should have an effect but it is possible that it has a different ingredient in it the fish are reacting to or some such?
I assume you were exact in your amounts of treatment for the tank and didn't overdose them?
Bullying can be fatal. I've had three incidents of it. Two which ended in the death of a fish. And I didn't witness any actual bullying first hand. If one is a bully try taking him out of the tank and putting him elsewhere and see how things go.
Sorry I'm not much help. Best of luck.
Hugs,
P.