Hi guys and thanks for all the responses.
As for the aquarium being slightly overstocked, I am aware of that now
(the guy at the pet shop said that I could buy one more!). All the swordtails except one are not mature I am thinking of finding somebody in London who keeps Otos so that my Oto could get company and be happier (also the mollies that I bought have ich now and I'm treating it with salt so I don't think he likes it).
I have a Tetra testing kit, plus a separate Nitrate one (and soon an oxygen test and hydrometer arriving through the post) so I'm keeping a close eye on the conditions. I used to have issues with high Nitrate but I realised that the filter was not working properly and now things have improved.
Also, I have two air-tubes with air-stones and I tried installing an air-disc but no success, I guess the air pressure is not enough for all 3 at the same time.
I suppose that if I keep testing and observing the conditions, I'll be able to prevent them from dying...