Every vegetable we plant at Forgotten Forty Farm is heirloom, organic, and delicious, but what we're really known for is our tomatoes. 
Every year people in the area anticipate our nearly 50 varieties of heirloom tomatoes, and inevitably ask to purchase some plants for themselves. I don't have a great deal of space in my indoor grow-boxes for many more than the 600 I have to start for myself each year, and I won't have a heated greenhouse until the hobbit house is finished, so the problem presents itself; "Where do I start plants for everyone else?"
I clone my tomatoes and I use my fish tank.
When I start the process, to save space initially, I plant my tomato seedlings 5 to a four inch pot, in a dice pattern. I let these seedlings get to a transplantable size, with at least 2 sets of mature leaves and a decent growing tip, and then I separate them into individual pots. This usually occurs for me at about 3 weeks after sowing. When they are transplanted, they are dropped down in the soil to the first pair of true leaves as a means of supporting a stronger root system, and buying me more time under the grow lights in the space available.
My shelves are set up to support a maximum of 18 inches under the lights in order to give me as many shelves as I can manage vertically, while still having room to move the lights up and down as needed to promote good growth of the plants at any size. When the plants reach their maximum height for the shelves, it's time to clone. I find that mine usually take another 2 weeks after transplant, but every system will vary. The important thing is to make sure the plant is sturdy, healthy and at least a foot tall with a minimum of 4 pairs of true leaves at a good size, and a well established growing tip.
When cloning, I prefer to do a coppice cloning, leaving two leaf petioles on the mother plant to form 2 new growing tips. I string my tomatoes vertically, so this gives me a great base to start from when I plant them out, and also gives insurance in case one of the growing tips doesn't take. If I end up with 2 growing tips, I wait until they are old enough to clone again, and I take another cutting of just one sucker. This way I maintain my single growing tip for the plants I'm keeping, while still being able to clone for sale to the public. When I take the cuttings, I make sure that there is at least one leaf on a stem, and that the stems will be long enough to stick into the water (about 3 inches minimum).
The aquarium is a 90 gallon tank set up with a HOT (Hang On Tank) filter, and a jet pump filter to help circulation. It is heated to roughly 70 degrees, and has a single length of white wire closet shelving laid across the top of the tank to organize and support the cuttings until they root. Labels are hung from the lip extending down the front of the tank for easy identification, and grow lights are suspended above the whole system. I have Fiddler Crabs, Ghost Shrimp, and Apple Snails stocked in the tank for cleaning. The Apple snails are especially beneficial, as they eat decaying plant material and algae only, not live plants, so I'm able to clone in the tank, and they help tend to any clones that fail to strike for whatever reason.
With the Apple snails cleaning up the edges of the cuts that would normally die off, along with any other problem pieces, I get a near 100% success rate. The clones usually start rooting immediately, with noticeable development in 2 days. They are almost always ready to plant by the first week, and take to soil very well at that stage. With this method, I'm able to save space in my grow-boxes while developing 250+ clones in the space that only 36 would normally fit.
I call that a win.
Happy Homesteading!