I am impressed with all of this help, cheers!
The tank is about 25 litres (?). I do a water change each weekend and had the water tested last weekend - It was just right.
There are 5 neons, 5 harlequins, 2 dwarf gouramis, 2 clownn loaches, now only 2 guppies, 3 platties + 2 babies, 4...