Loosing fish... any suggestions appreciated

I have 3 tanks on the go 60, 125, 260- this is only happening to 1 tank and I'm using that as a basis, along with testing to rule out changes in the tap water.

Main tank is a 260 litre and over the last few days I've lost a number of apistos and cardinals - yes some of the more sensitive fish. Testing has shown that ammonia, nitrite and nitrate are all perfect. Ph in the tank which is normally around the 6.7 mark had increased to just over 7 which I am putting down to the addition of an air pump about week and a half ago - which has now been removed in the process of elimination. This removal, and the water changes has brought it back down to 6.7. Temp was 29C.

Water change are done at least once a week and this was done on Sat morning as usual. Water changes does involve the vac of substrate. I do clean out the filters on a regular basis - alternating between internal and external overtime - note never a full wipe out in either filter - it's a sponge at a time.

My biggest concern is the Discus. 2 of the 6 have turned quite dark and are hiding in the back. However they do move quite well during feeding time - and are eating. All Discus however are unhappy. Not as full coloured as usual.

So, water would appear to be OK - however again as a matter of attempted remedy of the situation I have
a) added polyfilter to the external filter to take out those nastys that I can't see
b) Water changes each day - since Tuesday when I came home to the big kill.- and yes I bring water up to temp
c) Moved temp up to bring it to 31 from 29
d) Change the Carbon in the internal - not due for another 2 weeks but I'll try anything at the moment.

Any suggestions welcome.

Monty

Hi Monty,

You appear to be doing everything right. I don't think the movement in pH would have had any impact, especially over the short term.

If it's only one tank, I'm not sure that it's your water. Surely that would be impacting your other tanks also. Do you use RO for your Discus tank? If not, it's something you should consider as they are very sensitive to any changes in water conditions.

What are you feeding your fish?
Any new arrivals in the tank?
Any chances of a couple of pictures?

Any chance of them getting too stressed from the little people around the house ?
ie. Banging on the tank etc.

Processor.