what do you mean by filthy...algae isn't dirty. I'd be more concerned with whether or not the fish are diseased. My criteria for a good or bad lfs is as follows (in no particular order)
1. do they have adult sizes labeled
2. do the employees seem to know basic info about fish care
3. Are the fish healthy
4. How many floaters (dead fish) do they have
5. Do they stock dyed mutilated or poorly hybridized fish (if the mutation would prevent the fish from surviving in the wild then I'd call it a poorly hybridized fish)
6. How do the employees answer the following question. I'm having an algae problem what can I do.
A. You could try modifying your lighting or perform water changes to reduce your nitrates...other options include live plants or chems that remove phosphates nitrates etc...as a last resort or if your just lazy you could buy a pleco or some other algae eating species, by the way how big is your tank.
B. I've got just the thing come with me. What you need is a Plecostomus.
There's more. Just a short list. But really cleanliness isn't as important to me as a varied and healthy stock. Now algae on the glass is just bad business. If you can't see the fish clearly then you can't tell if it's healthy or not and I would not buy it. Algae is only a concern for me if it's a pest like beard or cyano bacteria. Then you should quarantine any plants you buy. Of course you should do that anyway.