Summertime Koi Pond Care
After the pond water warms up in the spring, the fish become more active and return to normal eating patterns. You can switch from a low protein "spring time" food to a higher protein diet that will aid in their growth. By summer, the fish should be fed two times daily, as much food as they will consume in 3-5 minutes. Do not overfeed, as uneaten food will decay and increase the toxic ammonia levels in the pond.
Summer will also increase the pond water temperature and the evaporation rate. As water evaporates from the pond in the summer, the dissolved minerals (calcium and magnesium carbonates) will remain behind. These minerals form the white chalky residue along the margin of the pond. Over time, they can build up to high levels causing the water to become very hard, resulting in an increase in the pH of the water. Any ammonia in water with a basic pH (greater than 7.0) becomes more toxic to fish. Consequently, it is important to not just replace water as it evaporates from the pond, but to drain out some of the high-mineral-high-pH water and replace it with fresh water. Always use a water dechlorinator when adding city water to the pond. Chlorine in the water will irritate the fish's gills and can lead to respiratory distress, secondary gill infections, or even death. If your city water contains chloramine, then you will need to add an ammonia remover as well as a chlorine remover to the incoming water.
If summertime temperatures are extreme (water temperature above 86°F / 30°C) the pond may need to be partially shaded. Warmer water not only holds less oxygen, but also increases parasite reproduction that adds to the fish's stress. Treat with appropriate medications if the fish are seen scraping their sides on rocks (flashing), have increased mucus on their skin, or are breathing rapidly at the water surface (due to low dissolved oxygen and gill irritation from parasites). Be aware that many medications may also lower the oxygen level in the water.
Oxygen levels are lowest in the morning, as aquatic plants will use oxygen for respiration at night. During the day they use carbon dioxide (CO2) for photosynthesis and give off oxygen (O2), but this process is reversed in the absence of sunlight. Be sure filters, waterfalls, and fountains remain on at night to oxygenate the water, even if no one is there to see them. High levels of carbon dioxide in the water from plant respiration may lower the pH of the water. Adequate water agitation from water falls or fountains will also cause dissolved CO2 to dissipate.
Summertime is a great time to enjoy a pond full of colorful fish, but regular care is needed to keep the water quality in the pond safe for fish and to keep the fish healthy and happy.