Generally speaking, you want to have as little algal buildup as possible. Algae thrive in areas with a lot of light (from the sun or a bulb), but they also use much of the oxygen that would typically be available for your fish. To reduce the likelihood of growing algae, ensure all tubing and tanks in your system are black or opaque so they block as much light reaching the water as possible. This sometimes also means covering your fish tank as much as possible, while still leaving room for oxygen to enter.
Ideally, you should have a backup power source or battery for situations like this. However, in the event that you do not, a less crowded fish tank means the fish can survive longer without extra oxygen. Be sure someone is checking the system daily.
If you have just started your system and put fish in, it may be the shock of a new environment that caused them to die. If your fish are used to warm climates but your water is too cold, that could also be a factor. If you have not added bacteria to the system yet, ammonia isn’t being converted into nitrate for your plants to take up; therefore, the ammonia buildup may have killed your fish. Overcrowding the fish tank can also stress out fish and cause them to die.
Ideally, you’d like to have about ppm oxygen in your tank for healthy fish and plants. If your dissolved oxygen is less than that, it may be due to the high temperature of your water (as temperature and dissolved oxygen have an inverse relationship) or an algal bloom. One way to increase your dissolved oxygen is by adding aeration pumps.
The water in your aquaponics system should be between 6.8 – 7.0; however, others have experienced success slightly higher/lower as well. If the pH is too acidic, add calcium/potassium carbonate (in the form of sea shells). Add in small doses. It is much better to make small but often changes rather than a large change at once. If the pH is too basic, add a very small amount of 18% phosphoric acid. The pH will not change immediately, so wait multiple days before adding more and checking the pH. Also, high (basic) pH can lock up necessary nutrients that your plants need to survive. This can cause slow plant growth or cause plant death.
In this case, it is recommended you add pure iron chelate to your water for the plants. It makes iron soluble in water and thus, more accessible for the plant. In addition, it is also used as a way to treat a plant that is producing insufficient chlorophyll.
Your fish need bacteria to break down their ammonia and make it less toxic, but your bacteria need your fish to feed them the ammonia so they survive! We recommend adding the appropriate amount of nitrifying bacteria for your quantity of water, and also adding small tilapia fingerlings (or just a few large tilapia). This will ensure your bacteria are being fed and are growing, but there won’t be more ammonia to convert than they can initially handle. As the tilapia grow and produce more ammonia, the bacterial colonies will be large and established enough to handle it!