Some reefers, like me, choose to buy their RO/DI and saltwater from a LFS. If a LFS has expensive water, then I will not likely go there, unless they have something specific I want in stock. I'm sure they all make a profit from selling water, but you might need to view water as a potential loss-leader: getting people into your LFS is the first step in them buying something and potentially turning into a repeat-customer. Many stores employ loss-leaders to get people to visit (grocery, clothing, car dealers).
Everything you sell should have some level of quality, even the "free" advice. Online reviews and word-of-mouth can put a big dent in your sales volume. Your RO/DI should have low TDS and no Silicates (personal peeve). Your saltwater should be consistent. A small variety of healthy fish is preferred to a large variety of unhealthy fish. If you run low-dose of copper to hide disease, people will learn not to buy fish from you and tell others.
Based on several LFS that I frequent, fish (fresh and saltwater) are the most profitable, based on the volume of the store that is fish versus dry goods. A variety of food (especially seaweed) and medications is favorable to me. Dead fish and expired food/medications/test-kits are a turn-off. If you carry SPS frags and have to run hot, bright lights, then a higher indoor temp in your store is acceptable to people who might buy some of those frags.
Honest, accurate advise is highly valued. That is another reason people will visit a particular LFS versus another. Again, once people are in your store, for whatever reason, they are more likely to buy something than not.
If you don't know the answer, "I don't know that myself" is a perfectly acceptable response. If it's a fish-compatibility question that you can quickly find, do it. Trust and a repeat-customer is more valuable to your business than a quick buck and a bad experience.
