After weeks of your students being excited about getting a fish, today is the day!

Word of caution–don’t add a fish for two weeks after building the initial column and for two days after building the aquatic chamber.  Seriously–don’t do it. I did not heed the warnings myself the first time I built and had massive fish death.  Your students will keep bugging you about fish, but teach them patience. The reasons why are found in my previous post-Building the Aquatic Chamber.

Student directions for building ecocolumns.

Read about buying supplies, cutting and filling bottles, planting seeds and taking data , adding bugs, worms and leaf litter, and building the aquatic chamber in previous posts.

What Fish to Add?

Let’s discuss some of the popular varieties of fish for ecocolumns. There are many bad choice and to be honest, I’ve used some or allowed students to buy and wish I knew this information. Shout out to Katy Sturges for some of the info on fish no-nos.

Goldfish: Cheap, but not good for ecocolumns. Their waste contains a large amount of ammonia which will burn their scales (black spots). They will also eat all of the elodea leaves quickly and kill it. You may need to replace the elodea or supplement with fish food.

Rosy Red Minnows: Feeder fish so they’re cheap like goldfish, but I’ve found that they have a high mortality rate. Since they’re so cheap, you can replace easily when they die (Dead fish teach a lot about nutrient cycling when students bury in their soil).  Will sometimes eat a lot of elodea too.

Beta (Chinese Fighting Fish): Some people use in ecocolumns, because they can gulp air from the surface and will survive when dissolved oxygen levels are low. The problem, however, is that they are insectivores and will only eat the elodea if desperate. They need a supply of brine shrimp, zooplankton, daphnia, or mosquito larvae. If you use healthy pond water with these invertebrates, this might do the trick.  The other problem is that they can see each other in adjacent ecocolumns which stresses them out. There are, however, creative ways to remedy the problems and use beta fish.

Guppies:  A good choice, but can be pricey if you have a lot of columns. They can tolerate cold and dirty water and aren’t too large to use up the dissolved oxygen. Some of my students purchase these at the pet store.

Gambusia (Mosquito Fish): My favorite. My original population came from Los Angeles County Vector Control which donated a bunch. Vector Control uses these fish in abandoned pools or ponds to eat mosquitos and their larvae (hence the name). Can also be purchased from Carolina Biological, but they have a hefty price if you include the shipping cost.  If you purchase some or get some donated, try to keep year-round. They will breed and keep your stock going. I have a super strong stock now as the weak ones have died off and the strong keep breeding. Natural selection at work! A draw-back to Gambusia is that they are omnivores and prefer to eat invertebrates. But, they will eat the elodea and survive in Ecocolumns for several weeks.

Tetra: I’ve heard good things about these fish from other teachers, but have never used myself. Can also tolerate cold, dirty water.

Snails: A good solution if you can’t find any healthy fish to add. Snails are often pests at the pet store and they will give them away to you. They will eat algae.

Take Water Quality Data First

Students should take water quality data first before adding a fish. They need to see what changed in their water after the elodea has been in the water for a couple of days. Students also should take other data–plant height, soil etc.

After taking water quality data, students can add a fish. Make sure the dissolved oxygen levels are higher than 3 mg/L before adding a fish. They should be if students cut some flap/air holes in the chamber.

Adding a Fish

As a rule of thumb, students can add up to two small fish (guppy-sized) or one medium-sized fish (1-2 inches). If they add more, they run the risk of killing them all due to dissolved oxygen depletion.

This is the last stage of building the ecocolumn. I try to have students take data again within a few days to see nitrates building up in the water and then weekly after that.