Hi, always willing to give a LFS a chance but dead or poorly maintained tanks is unforgiveable, always try to find out when a shop is receiving new stock, they normally have set days in the week. They should be quarantining fish for a few days after they have been received just to make sure they have no problems. Have always had a rule if I see dead fish or livestock I will not buy from that shop. Sounds harsh they have to be responsible for their livestock, always ask them questions that you know the answers to, can I put a particular fish with X or is this filter going to be correct for my tank. Staff in shops vary, and I do not accept the excuse of being too busy, its their job. If you get the wrong answers then you need to weigh up the risk. Don't be fooled by multi-buys like buy 10 fish get 2 free!! The two free may be too much for your tank, it's not a good deal if you end causing more issues. Look for quality not quantity how bright are they, are they moving well, check fins and body for splits and marks. I would rather have 5 quality fish than 7 poor or dull fish. Get to know the staff and if its a local one off trader the owner. I was in a shop and about to buy some beautiful Pictus and a customer in front of me wanted some as well, I watched in horror as the assistant went and got a net and tried to catch these, and I had to say stop, you will injure these fish use a something to coax them into the bag, but never a net. When I asked he had not been told that he could break their pectoral spines in the fins. I never shop on a weekend for fish there are too many people around to get served correctly and you also get weekend staff that are not always the best trained. Take your time and if in doubt don't buy.
Hope this helps.
Brecon