I'm guessing a monopoly doesn't necessarily come from being the sole distributor. The monopoly could come from the distributor inflating the price of a product to the point it's no longer viable for retailers to buy and sell, since there is little to no profit in doing so.
This way the distributor becomes the retailer as well, retaining the entire market share, shortening the supply chain, and gaining max profit. It perhaps has its benefits as a business model for a sole distributor in a niche market offering specialist goods. But it's also potentially short sighted in that market reach and penetration can be severely limited and in turn so can growth.
I'm also guessing this can leave the business vulnerable to competition. Which in this case may well have happened especially with other manufacturers of quality aquascaping equipment successfully breaking in to an expanding market. But who really knows?