There should be no restrictions or discrimination in deciding who can use the services of the co-op as long as they are willing to accept the responsibility of membership. Keep in mind that a "member" of a co-op is actually an "owner" of the co-op. The price of ownership is a fixed amount, which is typically much lower than the "book value" (get out your old accounting text) of an ownership share. 

There is a huge difference between buying an ownership share at a fixed price and buying shares of a for-profit corporation at the going market price. The market price of a share of corporate stock often has little, if any, relationship to the book value of that share of stock. For example, eBay has a market value of $150 to $200 per share but a book value of only a couple of dollars per share. The end result of this dichotomy is that ownership of a corporate share is "money-based", but ownership of a co-op share is "people-based". 

Much like an aquarium society, a fish hobbyist co-op should definitely be a people-based (and fish-based) operation. As long as the co-op is able to pay the bills (including any salaries), the focus should not be on money. Anyone should be able to join to co-op, from the fishaholic with 100 tanks to the person who owns a half gallon goldfish bowl.