A good knowledge of lots of different aquarium products, and which ones are appropriate for solving customers problems would be a good thing to mention.
Also, don't just say that you know about fish and the like, simple things like being tidy up as you go. Polishing tanks and de-algaeing while you water change, and knowing that it important not to leave clutter (like buckets during a water change) lying around are fairly important things that rarely get mentioned, even though they are extremely relevant to the job.
In other words, mention things you know about fishkeeping, but don't focus on things like fishless cycling, how fast carbon is spent or the poor survival rate of ottos in captivity. Focus on how good you will be at selling the right products, and making the shop run efficiently.
If you have experiance in retail before, saying that you are very friendly and active around customers would be good too - but mention that you don't waste time with customers, and can get on with a job while chatting to them. Being friendly is great, but there is always something that needs done when your not actively selling something.
Been working Saturdays (and 5 days a week most holidays) at the LFS for...3 years now? Started full-time a few months ago.