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Learn how to care for freshwater community fish, recommended diet & aquarium setup. Find the right food, supplies & equipment at your neighborhood Petco.
Updated on November 13th, 2025

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Small Marine Fish are favorites with marine aquarists and includes a wide range of species.
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Visit Petco to learn about cleanup crews and how they can help your aquarium health.
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The quick start liquid will add nitrifying bacteria but will not eliminate ammonia or nitrites. Both are harmful and can kill fish in high enough concentrations. You can use a home test kit to test the water or you can submit a sample for testing. Most pet stores or aquarists will do this for free or for a nominal fee. There are chemical additives you can use to decrease levels. When adding water, use treated water or aged water. Ideally, the tank should be left empty for about 4 weeks to properly establish the nitrogen cycle before adding fish. At this point, you may lose the second fish regardless of treatment. Test the water, make adjustments, let the tank cycle and start fresh. Adding an airstone can increase oxygen levels.
Updated on August 12th, 2025
This could be bacterial or parasitic infection. It is difficult to determine definitively since the image provided is not clear. Fish can be difficult to photograph. Basic care includes checking water quality and chemistry. This can be done using a home test kit or by submitting a water sample for testing. The local Petco may do this for free or for a small fee. There should be an absence of ammonia and nitrites with nitrates at 20-60 ppm. pH should be as close to 7 as possible, avoiding large swings. Koi can tolerate a wide range in temperature, but generally do best between 65 F and 75 F. Again, avoid large swings in temperature to prevent stress or shock. If necessary, a partial water change can be done to stabilize chemistry and/or improve water quality. If you have other koi, consider placing this one in a hospital tank to make treatment easier and to prevent spread if the condition is infectious. Plain aquarium salt (NOT table or Epsom salt) can be used to reduce stress, impro
Updated on August 12th, 2025
Thanks for reaching out about your aquarium and including photos. Did you test your water quality prior to the water change? Continue to test for pH, ammonia, nitrites and nitrates. Pristine water quality is needed for health. Are any of the fish new, that could have introduced parasites? Itching/flashing can be due to water quality, external parasites, bacterial or viral infections and other causes. The white lesions on the photo could be fungal infections. A skin scrape and examination under the microscope (at a veterinarian's office) is needed for an attempt to determine a cause. Fungal infections are often secondary problems, so check water quality first. Good luck.
Updated on August 12th, 2025
These are quite generic but concerning signs. The list of differentials is very long in this case, so I would recommend a logic and radical approach. Begin by submitting a water sample for quality testing. Most pet stores will do this for free or a nominal fee. Clinical examination should always include a water quality check, even if you have done so already. Water testing should be performed with a quality test kit, measuring NH4, NO2 and NO3. High ammonia levels, PH imbalances may be responsible for these signs. Oxygen concentration and quality of the filters should be determined as well. If water quality is good and there are no major imbalances, your fish should get examined to look for any sign of bacterial/fungal infection and eventually water should be treated. Hope this answer was helpful, but please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further. Please find the time to give us a feed
Updated on August 12th, 2025
If persistent this is a very concerning but generic sign, which requires a logic but quite radical approach. Begin by submitting a water sample for quality testing. Most pet stores will do this for free or a nominal fee. High ammonia levels, PH imbalances may be responsible for that. Oxygen concentration and quality of the filters should be determined as well. If water quality is good and there are no major imbalances, your fish should get examined to look for any sign of bacterial/fungal infection and eventually water should be treated. Hope this answer was helpful, but please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.
Updated on August 12th, 2025