My Tetra fish keep dying. What's causing this despite my best care?

I'm sorry you are having difficulties with your aquarium. There are several issues that should be dealt with right now. They are water chemistry, aquarium size and choice/number of fish. Any new aquarium should be set up and, ideally, run empty for several weeks (usually 4-6 weeks). This is so beneficial bacteria have an opportunity to populate the substrate and filter. These bacteria are necessary to break down wastes that fish will produce. The technical phrase for this is "establishing the Nitrogen cycle". These bacteria will break down wastes into ammonia then nitrites then nitrates. Nitrates can then dissipate or be absorbed/used by live plants. In high enough concentrations, ammonia and nitrites are toxic and can kill fish. Cycling a tank with fish is possible but it increases the risk of loss. Water chemistry should be checked every few days using a liquid home test kit. A water sample can be submitted for testing , but is not as convenient as testing yourself. Most pet stores or fish stores will do this for free or for a small fee. Partial water changes, no more than 30%, can be done as needed to stabilize chemistry and maintain water quality. Choices for water conditioners include NovAqua, AmQuel Plus, Prime, StressZyme or StressCoat. Tank size is another concern. With most fish, the widely accepted "one inch of fish per gallon" will be sufficient. For koi and other carp species, including goldfish, the rule is not sufficient. At full size, goldfish need 20 gallons per fish with an additional 10 gallons for each additional fish. Koi do best in larger tanks, much larger...we're talking in the hundreds of gallons or small ponds. These species produce a larger than average bio-load (wastes) that can quickly affect water quality in a tank. To combat this, large water volumes and powerful filtration are required. Any filter should be rated for at least twice the tank volume. I'm not sure what size tank you have now, it may be fine if the fish are young and small, but they will quickly outgrow it. The last issue is choice of fish. Koi and goldfish are cold water fish. They do best in water between 65F and 72F. Most species of sharks do best in the high 70sF to low 80sF. Keeping fish in incorrect water temperatures will lead to stress and illness. This could be compounded by the water quality/chemistry issues you are having currently. Until the chemistry is stable and water quality is maintained, your fish will continue to become ill and possibly die. Any treatment will not be successful in this situation. Do not purchase or add any more fish until the tank has cycled. If you have additional questions, you can post them in the forum or request a consultation to discuss the case in greater depth.

It is possible the tank has not cycled properly which results in high ammonia and nitrite levels which can kill fish. Allow the tank to run, without fish, for at least 4-6 weeks. During this time, use a home test kit or submit a water sample for testing every few days. The local Petco may do this for free or for a small fee. What you are looking for is to have ammonia and nitrites at 0 ppm and nitrates at 20ppm or less. pH can hover around 7 for most community fish. Avoid large, rapid swings in pH though. What is happening in the tank during this time is that beneficial bacteria are populating the gravel/substrate and the filter. These bacterial populations are responsible for the Nitrogen Cycle. This cycle breaks down wastes from fish, uneaten food and plant matter into by-products that are filtered out or dissipate. You can "cycle" a tank with fish, however this increases the risk of loss. Once your tank has cycled, you can add fish. You will still need to monitor water chemistry and perform partial water changes regularly. Use only conditioned water that is temperature matched to prevent stress or shock. You will also need to vacuum the gravel regularly and rinse the filter media. Never use tap water to rinse. Use a bit of tank water to protect bacterial populations in the sponge.

Thanks for reaching out about Diesel. This surely points to water quality issues. Have you tested the ammonia, nitrites and nitrates? Any chance of copper in the system? Parasites and bacteria rarely cause death this quickly. Remove all fish and place in new tanks, with spring water (bottled or reverse osmosis). Change or add carbon filtration. Good luck.

Loss of color in fish can be a sign of severe stress. Stress can be brought on by disease, poor water quality, over cleaning the tank (too frequent or too large water changes), over aggressive fish, poor or inadequate nutrition, being moved, over crowding, and a host of other situations. I recommend that you start by reviewing your water quality and tank environment. How are your water parameters? Review how often you do water changes and how much water you change at one time. How much are feeding? Could you be over or under feeding your fish? Is there a bully fish in your tank - a fish that chases or aggresses against other fish. Are the number of fish in your tank suitable for the size of the tank? Look for signs of disease. Fish can suffer from bacterial, viral, fungal, protozoal, and parasitic infections, but not all species of fish suffer from the same illnesses. You can find charts of fish diseases in books or on the web to help you narrow down what illnesses might be affecting your fish. Treatment of disease will be based on determining what type of organism is making your fish ill. To prevent illness in fish, maintain high water quality, avoid over crowding your tank, and quarantine new fish before introducting them to your tank.

It is hard to tell from the picture if your fish has fungal plaques on the scales or if this is normal pigment. I would recommend taking your fish to a vet for a necropsy. That will allow for biopsies and cultures to be sent to the lab to determine the cause of death. IF it is fungal disease I would recommend treating the tank and cleaning the environment well.

If left untreated, ich will eventually kill. It can weaken the fish after which infection will set in and finish it off. It is also possible the fish were killed if a water change was done as part of the cleaning. Aged or conditioned water should always be used to avoid exposing fish to harmful chemicals, mainly chloramines, many municipalities use in treatment. Water that is not temperature matched can cause shock which will also kill fish. If can be treated by gradually raising the water temperature over 24 hours to about 85 F. Ich is temperature sensitive, this the warm water will speed up the life cycle. Once the "dots" fall off your fish, vacuum the substrate and then use a commercial ich treatment. Leave the temp at 85 F for 2-3 weeks to clear the tank. Afterwards, gradually decrease temperature to the proper temperature for your species of fish.

I am so sorry that this is happening, and I am sorry for the loss of two of your fish. With you mentioning the filter stopping, there could be a number of issues going on. Filters perform a number of tasks, depending on the type you have. First, they help to keep toxins (ammonia, nitrites, nitrates) from building up in the water through the support of good bacteria - when the power goes out, sometimes the bacteria can die. This can put a lot of stress on the fish. Also, the oxygen levels can decrease when the power goes out (not sure if this occurred for you or not) and this can cause fish to die, and others to get stressed. When stress occurs, sometimes fish will injure themselves if they are panicked and trying to move around the tank. From the pictures, it looks like the reddened spots are under the skin and could be related to trauma. There are other issues that can result in spots that look like this as well. I would recommend getting your filter started back up, doing a water change, and then testing your water quality. Once it is back to normal, you can put your fish back in the tank. If your filter didn't stop due to a power outage, I would recommend considering replacing it, or attempting to find the underlying problem that resulted in its malfunction in the first place. For the reddened spots on the head - if they are due to trauma, they should resolve over the next week or so. If you see progression, or if there is a lack of improvement, then I would recommend taking one of the fish in for evaluation by a veterinarian, as there may be something else going on. I hope that this gives you a place to start, and good luck!