Coralife Aquarium Maintenance

Mastering Soft Coral Care: Setup, Feeding, & Water for a Healthy Reef

Learn how to care for soft coral, recommended care & habitat setup. Find the right food, supplies & equipment at your neighborhood Petco.

Updated on April 9th, 2026

Care Sheets

Your FOWLR Marine Tank: Easy Setup Guide for Live Rock Fish

Learn how to care for Marine FOWLR Aquariums. These tanks have easier upkeep and require less testing and supplementation than a reef tank. You can keep many fish who would generally consume corals and invertebrates, making it a popular setup choice for more aggressive fish.

Updated on October 23rd, 2025

Care Sheets

Anthias Care: Set Up a Thriving Tank for Vibrant, Healthy Fish

Learn how to take care of Anthias, recommended diet and tank setup. Find the right food, supplies and equipment at your neighborhood Petco.

Updated on April 9th, 2026

Care Sheets

Clownfish Care: Your Easy Guide to Tank Setup, Food & Compatibility

Learn how to care for a Clownfish, recommended diet and aquarium setup. Find Clownfish for sale, supplies and equipment at your neighborhood Petco.

Updated on April 9th, 2026

Care Sheets

Cardinalfish Care for Beginners: Tank Setup, Feeding, & Habitat Tips

Learn how to care for cardinal fish, recommended diet & aquarium setup. Find the right food, supplies & equipment at your neighborhood Petco.

Updated on April 9th, 2026

Vet Q & A

Why are my fish dying with bleeding, white spots, and polluted water?

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.