I'm a beginner and am just looking for help and advice :)

Dangalf

Hi everybody,
First I would just like to say that if I am doing something wrong, I am sorry; I am not trying to intentionally harm or kill my fish, I just don't know. I thought that I had read up enough on the subject beforehand, but I am starting to doubt that, as well as the reliability of the advice I was given by my local pet shop.
Now on to the 'technical' details. I have a 30ltr tank and have 10 fish in there (my pet shop said that I could have 12 - 20 small sized fish; guppies and neon tetras), 4 guppies, 4 neon tetras, 1 marbled hatchet fish and 1 otocinclus catfish. Along with the fish are two real plants, one ornament with three fake plants on, a thick layer of gravel, a light, a heater and a filter with a tube that draws air in. My tank temperature is 25/26 degress celcius, the pH is 7.2, the ammonia level is 0ppm, the nitrite level is 0ppm and the nitrate level is 10ppm. I feed them a couple of flakes per fish (about 20/25 flakes in all - crushed up, nice and small) twice a day. I change 25% of the water every month (which I get out using a gravel vac) and when I replace the water, I add warm water, the instructed amount of 'Nutrafin' Biological Aquarium Supplement: Cycle (to help boost the bacteria), the instructed amount of 'Kordon' AmQuel + (which removes Nitrate, Nitrite, Ammonia, Chlorine & Chloramines, the instructed amount of 'Pets at Home' weekly water cleaner (which removes ammonia and nitrite), the instructed amount of 'Pets at Home' tap water treatment (which removes chlorine and chloramines). I can't really think of anything else to tell you. If there's anything that I'm doing wrong, please say.
I have actually lost 10 fish in the last 5 months, but I think that some of the deaths were down to the fact that I put cold water in after a water change (it seems silly in hindsight, but at the time I didn't think). Does anyone know why they would have died? When a fish dies of old age, what happens (because a couple I think have died of old age, but they've gone limp and just floated aimlessly around, still breathing, before dying).

Thanks so much in advance, I know it's been a long post, but ANY help or advice that you can share with me, will be greatly appreciated.

Dan.