These mesocosms are simple: no water circulation and no electrical power. That means you have to manage them wisely. For example, you can only add a half-dozen or so small goldfish, and you can’t feed the fish too much, or the water will turn murky and green.
Our recipe for success. Not too much food, some plants, and enough shade. AND Practice and Learn.
We stock each pond with a water lily in a pot, holding the soil in the pot with small stones or a brick. The water lilies absorb nutrients. And the water lilies often create enough shade to reduce algae growth. If the water lilies get too big, we take them out, remove half or more of the plant, and repot them. Often, we add other water plants in more pots. We do not add invasive species!
We watch for water clarity. If the water is turning cloudy or green, that’s a sign of too many nutrients, like a polluted river or lake. With too many nutrients, algae and bacteria flourish, blocking light for other species and removing oxygen. The pond may fail. Three solutions: (i) Feed less, (ii) Create more shade (for example, temporarily cover up to two-thirds of the pond with a board or boards), and (iii) if necessary, Change some of the water. Changing the water is often the fastest fix: use a bucket or a water pump to remove half or more of the water, and refill. (Usually, there is so much organic material in the pond that you don’t need to worry about neutralizing the chlorine – any chlorine quickly reacts with algae, bacteria, or the bottom layer of sediment in the pond.
Every fall, leaves accumulate in a pond. We take them out with a net; if there are too many leaves, we empty the pond (putting the fish and frogs into a bucket), and refill.
We experiment. With each pond, the conditions are a little different. If things go really badly, we learn from our mistakes, empty the pond and start over, and hope to do better next time.