How to treat dirty tap water with iron for a healthy aquarium?

RO water is devoid of minerals and trace elements that help stabilize pH and other parameters. If you can add these after filtration then use your RO water. Otherwise, RO is commonly used to "top off" fish tanks that lose water via evaporation.

Fresh tap water can be left sitting over night and any chlorine in the water will evaporate. It can then be used for your tank. Depending on the type of fish you have you should adjust pH and hardness of the water and of course temperature. Your local pet store can help you with that.

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.

Water softeners leave very little salt residue in the treated water. Most, not all, fish *should* do fine in softened water. Make sure you get species that does better in soft water. Use a hardness kit and monitor KH. This parameter is what can affect pH. You can set aside a gallon or two and test it after it has aged for 24-48 hours to confirm it is safe to use. Check the softener unit for a bypass valve. Some units will have one or you can get one installed. Taps/faucets outside may not be in line with the softener, so you can use water from there if necessary.

The best and safest method is through daily partial water changes, about 30-40% of total tank volume. Use only aged or conditioned water that is temperature matched to prevent stress or shock. Recommended water conditioners are AmQuel, NovAqua and StressCoat. These can be purchased online or in most pet stores or fish stores. A home test kit should be used to monitor levels. If a tank is properly cycled, beneficial bacteria will break down these toxins. Overfeeding, excessive light or heat and dead plants can cause spikes in ammonia and nitrite levels.

Monitor water chemistry using a home test kit or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Perform partial water changes to stabilize chemistry and/or to improve water quality. Use only conditioned water that is temperature matched to prevent stress or shock. Pristine water quality is the foundation for healing and recovery from infection or injury in fish. You can use plain aquarium salt (NOT table or Epsom salt) as a general tonic to reduce stress, improve gill function and aid in recovery. Dissolve 1 teaspoon of salt per gallon of water in a cup or two of water and gently pour it into the tank. Treat for 14 days then reassess the condition. An "all in one" product, such as Lifeguard (by Tetra), can be used concurrently to treat infection or parasites. Find one in the aquarium section of the local Petco. Follow label directions carefully and remove the carbon filter insert during treatment if indicated.

If you are getting hard water deposits, use a utility razor and gently scrape the deposit off the glass. When you're doing a water change, if the water level drops below the deposits, use a cloth dampened with plain vinegar and scrub. Be careful to not allow any vinegar into the water. Wipe it well with a clean cloth to remove any vinegar residue and refill. Ideally, the tank should be emptied, sprayed with vinegar, sit for 10-20 minutes then scrubbed or scraped.