We have raised fish before but never Catfish in our Greenhouse tank. Algae has always been an issue but with Catfish I'm not sure what we could add to the tank to help keep it cleaner... Suggestions?
Hey Lindsay -
It's a water quality issue. The more nutrients in the tank (old food, fish waste,...) the more algae you'll have. If you can reduce the sunlight available down to near zero, then the algae level will go down. However, you still have a nutrient load issue. I love a good problem to explore with students and apply the scientific method, so here is the path that I see. With your students doing the leg work set up the tank and get things operational including sampling and recording of the water for DO, pH, nitrates...daily. That way the students can watch the tank cycle once the fish are put in. Also a general observation of sunlight entering the greenhouse (Full sun, half day, few hours...). These readings will be your control measurements. Have your students brainstorm ways to reduce the algae and nutrient load based on what they know about algae, sunlight, and nutrient load. Some examples: styrofoam cover for the tank lid, same cover with hydroponic lettuce 1 plant per square foot, same cover & lettuce at 1 plant every 0.5 square feet, add a 55 gallon stock tank full of gravel to serve as a biofilter and so on. I'd go with the easiest to implement ideas so that you can give each solution 2 weeks to show any measurable improvement. Any idea is worth trying as long as fish health and student health aren't compromised. Then compare the trials to each other as you go. The goal is to reduce algae coverage and reduce the nutrient load including water clarity.
If that's more than you want to tackle with students then eliminate sunlight to get rid of algae. Then you have to expand your tank surface area a bunch to create spots for beneficial bacteria to grow and consume the nutrients floating in the water. Bioballs, gravel, or any small, porous structure with a lot of surface area will help. The higher the stocking rate of fish, them the larger biofilter you need. Think 55 gallon drums of gravel as 1 biofilter. I've seen University aquaculture facilities with 1500 gallon settling tanks for a small volume of fish but their water quality was perfect. Their fish tanks were in cinder block buildings with zero sunlight and low artificial light conditions. It's an art as much as it is a science, but it can be done.
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