Starting garden plants indoors allows you to grow your favorite ornamental or vegetable seedlings for much less expense than purchasing nursery seedlings. Keeping seedlings warm and well-hydrated can be difficult indoors, as lack of light and dry heated air tends to dry out the soil and weaken the plants. Make a small greenhouse out of an old aquarium to maximize the light in your home and provide seedlings with a high-humidity environment.
Cut a sheet of plastic cell-packs to fit inside the aquarium. Cell-packs are small, connected seedling pots that are readily available at garden centers.
Place a 2-inch layer of gravel inside the aquarium. Set the cell-pack sheet on top of the gravel. The gravel elevates the cells above any excess water that drains out so the soil doesn't become overly soggy.
Fill the cell-packs with a moist potting mixture. Use a sterile, soilless potting mix formulated for seed starting. Water the soil in the cell-packs until the potting mix is evenly moist throughout.
Sow two seeds per cell-pack at the depth recommended on the seed envelope--generally at a depth twice their diameter. Sow seeds that require light for germination directly on the soil's surface.
Place a sheet of glass over the top of the aquarium. Alternately, cover the top with plastic wrap. Set the aquarium in a warm spot. Seeds should germinate within seven to 21 days, depending on the specific variety.
Set the aquarium under a fluorescent light fixture that holds either one grow-light bulb or has both a warm-white and a cool-white bulb in it. Move the glass or vent the plastic wrap so one corner is exposed. Leave the lights on for 12 to 14 hours a day, or twice as long as the recommended light requirements on the seed packet.
Water the soil in the cell-packs when the surface begins to dry. If heavy condensation builds up inside the aquarium, remove the glass or plastic covering for one day to allow the excess moisture to evaporate.