My neon is REALLY pale!The blue and red stripe are hardly noticeable!WHY?? Lost 3 neons already!!?

I have a fish tank, tropical, and having never had fish…I was quite chuffed! It is a friends, and we’re renting his flat, and the tank was there with fish when we moved in! It’s got a big cat fish, a wierd sucking fish, with spots, 3 pirhana type fish (looks wise), but 1/4 of the size, and are a little aggressive….got tiger like markings on them (I dont know what they are) and 2 neons…..I went to Pets are Us, and got 2 albino cat fish, 2 neons, and 2 harlequens…..followed instructions of introducing them to the tank & water…12 hours later, I found them dead under a rock =-[ -gutted- !! I took them back to the shop the next day (feeling terrible about my dead fish!), with them in the bags in which I had purchased them in, with the tank water, which was tested, and everything was waay too high, apart from the HP. we got all the stuff to treat the tank,were advised not to buy any more fish until all had gone down, and have done 1/3 of the tank so far (it’s a 230 litre i think). Since then, I have lost both harlequens, 3 of the neons and now the last neon is pale and not looking good! =-[
Is my last neon’s fate sealed?? Or is it a disease??Maybe it’s beacuse he’s the last neon, and they need others?? Please help, I’m worried, as I have always wanted neons since i was little, and am now wondering if tropical fish are out of my depth??? There is soo much stuff to treat fish and the tank…I dont know where to start, and have also heard that where I got the fish from, is well known for it’s pets dying…your opinions and advice is mucho appreciated, I think I’m going to loose my last lil neon before the night is through….How have I managed to kill all of my fish?? I know the tank being too soiled wasn’t my fault, as i have only had them a couple of weeks…and the guy in the shop said it was probrably years since they had their tank treated…..But I want to stop this in the future.
Thankyou!