Thanks everyone for your input, help and all information handed over
Unfortunately all my cichlids have died.
2 Months ago when i took the fish into my hands, for the first 3 weeks there was no problems fish seemed to be perfectly normal.
the next weeks were horrible, every day or second day one would die then another until no more were alive.
The symptoms were basically heavy breathing ~ some would be swimming normally and have no problems, others were stuck mid water flow and basically looking like they were in a coma.
I understand everyones opinions in the probable cause.
Ammonia due to over feeding: The could of been one reason, but without feeding the fish for a week; some fish came back to perfect condition for a period of 4-5 days, with no problems, which makes me question ammonia poisoning as what i have learnt is that it permanently effects the fish so, i don't see how they could be looking healthy again, chasing each other and basically looking 'cured'.
Additionally, i feel that ammonia poisoning would have more of an effect on clown loaches and my bristle nose's and Gold spot Pleco.
Chlorine poisoning: This could of been an initial problem but seeing their perfect health for 3 weeks with no symptoms makes me believe it wasn't a problem, i did however treat the water several times in weeks leading up to the introduction of the cichlids.
nitrates/Nitrites: I took my testing kit to my NFS and it tested perfectly exactly the same as theirs. knowing this my water tested 0 nitrites and 0 nitrates, maybe this was due to my double filtration system. I understand that this is not 'normal' and somewhat 'bad' for the fish as good bacteria had not built up in the filtration but: knowing that they were perfectly fine in the first weeks makes me concerned to why it would effect them, if they were living under neutral conditions for this time with no problems.
WhiteSpot/gill flukes: I did in my efforts try to fix this as some fish i noticed were missing flecks of scales etc, however the fish didn't to make any recovery, but i don't think it made them worse, in fact after turning on the UV filter and treating for whitespot, the 'whitespot' which i identified on my clown loaches was healed and completely fixed, hence why i believe they are still swimming around happily.
Oxygen: the tank consists of 3 airstones which heaps of surface breaking, as well as a good power head built for this size of the tank in taking some air and breaking the surface with flow.
Overall, i do take it as being my fault for not caring for these fish, but i did try in efforts to make them live.
My conclusion is the substrate, i feel that coral sand has made a direct effect in the salt contents, and possibly build up of bad gases. I don't totally understand how this is possible, and/or if this has happened to anyone else.
It would however make sense that clown loaches and bristles were not effected as they are 'stronger fish', maybe this could of been the reason for ammonia, i am unsure.
Thanks again and hopefully someone can drop some input into coral sand, i am now just going to change substrates and start a common tropical tank for simplicity and looks.