My fish are dying .help?

i have a 240ltr tropical fish tank for 7 months now.9 congo tetra,6 juilie cory cats,2 bristlenose cat fish,15 platies,8 harlequin rasporas,10 neon tetras.all was well till 3 days ago when i noticed 2 harlequins dead.followed next day by 2 platties and 1 congo tetra . then today another congo a neon and a platy.my hearts breaking watching it happen as ive tried to do things the right way..i do weekly water change of 30/40 % and add tetra easybalance after a change.i have an external and internal filter both bio i change the top white pad each week but only clean all other media in tank water every 2/3 weeks.and dispose of my carbon every 2 weeks.i add declorinator to my new water and try to get the temp as close to my tank as possible.my tank temp is 24/27 degrees.i test my water each week .always seem to have omonia at .25 sometimes 0 .nitrites 0 nitrates 0 and ph is at 7.6..i have an air pump and a selection of false plants and hiding places .i have noticed some fish were gasping for air at the top 3 days ago .so as my fish book says i did a water change and one each day since but their still gasping and dying.i don't know what to do next .i know this means lack of oxygen but i have my air filter on except at night and my water change gives a little Relief but im doing something wrong .i was wondering if my tank was overcrowded as this seemed to happen soon after i added my final species .the harlequin rasporas.ive been conscious of this all along .help