Growing indoors with a hydroponics growing system is as enjoyable and rewarding as the weekend gardening enthusiast who cultivates the soil and care for there plants then can reap the end results of there fruits.

Here is a link that might be useful: More Hydroponics Information

Sorry I don't really have an appropriate picture to help your situation but I will caution you about putting plumbing fixtures over lighting fixtures. Be very careful to make sure the plumbing isn't going to leak or overflow onto the light below it. You might look into some form of trough system as well.

Also, 18" is not a lot of space for a light and a hydroponic system with growing plants in it. It might be perfect for starting seeds or if the system and light are really low profile and the plats you are growing will remain compact.
Though you do want florescent light very close (a few inches) above the leaves you need to be able to raise up the light to get easier access to the plants for stuff like harvesting.

Have you concidered using the shelving as in the center of a vertical growing system? Surround the shelving with lights and grow the plants on the shelves in the center? This might protect you from leaking nutrient from an upper system onto the lights below.

Well I scrapped the idea of doing it on the shelves. Instead I just used one to make a 2 shelved seedling growing area. I ended up using a 4 by 2 foot table I have and put 3 rubbermaid trays in it. I plumbed it underneath with swamp cooler bulkheads and made an ebb system. It seems to work pretty good. I got a switchable HPS/MH 400 watt light on clearance for $200 so All in All I have a nice seed starting area and rooom for 18 plants on my normal set up. In the end I ended up spending about $375 and am just starting to see my peppers sprout.