That sounds like a VERY cool job, especially when you're 15! I couldn't get a job when I was 15. I tried my hardest, but since I LOOKED about 10, no one would take me. It was all 'you have to have a national insurance card' even though most of my friends could get jobs... grrr
I worked in a couple of LFSs, the first, I was just cleaning tanks. The second I was cleaning tanks and medicating sick fish, and helping customers/till etc. I got a bit disillusioned though. It was all sell sell sell and if I was caught being honest 'you can mix tap and RO water together for a lot of fish, you don't just have to use just RO' 'common plecs get really big', I got in trouble. It was a lot of hard work, for not a lot of pay, and not a lot of thanks.
Your job sounds better though - you get to choose the fish!!!