GODZALLA;3553562; said:well sorry for the late ..
the tank is 110 Liter
fishes : group of adult angels + marbled red tail catfish + shovelnose catfish + severeum + high fin shark
i do only 25 w/c with no conditioning ( i know that it is hard to just put water from tap directly ) but i do that long time ago except in winter i have to heat the water to 27 C first so the heating Evaporate the chlorine ..
but i do that to all of my tanks and most of them is American cichlid and some cats and african cichlids
our tap water ph is almost 7.5
the temp is the same 27 @ both ..
but to be honest i do not measure the ph or the nitrates in tank as i do w/c every week
I know you say you've been doing this for a long time but i feel that this is the culprit. I've tossed tinfoils from tank to tank with 10+ degree differences, huge pH differences, and have done 50% WC on a 20-30g without issues. they are as hardy as they come.
I myself hate them because they're vacuums and won't let my other fish eat.
That large of a bio-load would be intense, and very very small room for error. By adding new fish, that are already stressed, use to clean water. Then exposed to fluxes of this capacity. even with a "cycled" tank the water itself is gonna go threw some major use/reuse and 25% once a week imo is not enough to keep it balanced. Likely your going threw some major swings. and your current stock is simply just use to it.