I'd like to weigh in on this thread. Rather than trying to quote anyone, I'll just ramble along. I've been selling and using (exclusively) Acrylic aquariums and
sumps since 1974. Once I bought and used my first Acrylic tank, I was hooked.
It's quite easy to scratch an acrylic aquarium, true! But, it is equally as easy to avoid scratching them. It's a matter of knowledge with an element of caring. Your
LFS employee obviously wasn't trained properly or didn't care or was ordered to "get-r-done" as quickly as possible and was careless or oblivious of picking up sand under the magnet cleaning tool.
Scratches can be taken out without removing the water. I've used this
Scratch Kit for years with great success, albeit with some labor of love. I just used this kit to take some scratches out of my boat, too. Worked like a charm.
Using Cuprisorb or carbon in a nylon bag in the
sump will work but, to increase efficiency a canister filter is best. One of the rare instances I recommend a canister. And, by the way, Acrylic and Weld-on are inert. They won't absorb other chemicals.
As far as "getting" anything from the
LFS for bumbling your tank, I think you're out of luck there. Most
LFS will balk at owning up to and doing something about an incident like that. I would talk to the owner/manager about a settlement. If they refuse, try threatening them with a law suit and if you know an attorney, have him/her give them a call. Doesn't hurt trying to get them to own up to their responcibility.
Good Luck!
Dick