Sorry no one seems to be able to offer a solution. This is a update of more info that might help.
I today took a sample of tank water to LFS
they tested it in front of me water was perfect no ammonia no nitrite and a ph of 6.5.
Bought 2 rosy barbs acclimatised them and apart from initial hour they were sluggish for rest of the time. They were put in tank at 1.30 pm and by 6.30 pm they were struggling to swim and were turning over did a PW of 30% no change, filled a bowl with water and moved them but they did not recover.
I assume that even with a 23Litre tank the water could not have been polluted in this time by 2 fish to the extent that the fish died.
I did after starting the tank from scratch all these weeks ago added a heater (which I did not have before) could it be at fault in that could affect the fish?
Please help I am on the verge of giving up on this tank.