Let them culture live fish foods. It has science (biology) in but don't tell them it's science based, and shows them how the life cycle starts in the oceans and rivers. Start with infusoria (Paramecium) and green water (single celled algae). Then move onto Daphnia and Rotifers, which eat the infusoria and green water. Add microworms, grindal worms and other small live foods.
If the kids learn about this they will be able to make their own live fish foods and see some weird little creepy crawlies that shouldn't freak them out, but will hopefully encourage them to look around nature and see little things, not just big things.
If you have a magnifying glass or microscope the kids can look at the food and see it close up.
The following link has info about breeding fish but tells you how to culture live foods. You can print it out (in parts or in full) and let the kids read it and follow the directions to make their own fish food. Maybe get them to do infusoria and green water first, then find some live Daphnia and give everyone some to put in their cultures. Let them grow the Daphnia in both cultures and see what they do better in.
STARTING OUT Make sure you have a pair, (1 male + 1 female). There is nothing worse than spending your hard-earned dollars buying a couple of fish to try and breed and ending up with a pair of fish of the same sex. Let's face it, to reproduce fish you need a male and a female. Two males won't do...
www.fishforums.net
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Teach them about using clean containers and not having soap or chemicals/ residue on their hands when working in water (basically it's bad for aquatic organisms). Give them all clean buckets with "FISH ONLY" written on them and teach them how to make a gravel cleaner from a plastic drink bottle and hose.
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You can teach them about the filtration cycle and show them different types of filters. Teach them how to clean/ maintain the filters.
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Teach them about water quality and chemistry. Water is made from 2 Hydrogen atoms and 1 oxygen atom. If you crack the bond in water you get hydrogen to run vehicles and oxygen to breath. If you combine them you get water to drink, use on plants or keep fish in.
Teach them about the pH, GH and KH. Maybe get some calcium based rocks and other types of rocks and do a vinegar test on them. The kids can put some vinegar on the different rocks and see what happens. They can also put the different rocks in containers of water and monitor the pH and GH changes in the water.
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Teach them about plants and lighting. Throw in some info about different wavelengths of light coming from the sun and from light globes.
Let them grow plants in water and in pots out of water. Most aquarium plants will grow on land, in pots or in aquariums. If they grow them in pots, the plants should flower and the kids can see the different flowers. They might even be able to get some seeds from their flowers and grow them on for a second generation of plants.
Once they know about the filter cycle and culturing fish foods, get them to breed some fish.