Oscars, and all aquarium fish in general are sensitive to ammonia, nitrites, long term exposure to high levels of nitrates, and large swings in pH and KH levels. You need to periodically monitor these levels, know if the results are appropriate, and if not, take appropriate action to fix the problem.
One common misconception about oscars is that they require a specific pH. This is simply not the case with the captive bred stock available in the pet trade today. In their native environment, oscars live in the somewhat acidic water of the Amazon river system. If you are dealing with wild caught oscars or low generation (F1, F2, etc.) stock, the fish may not thrive unless it’s kept in acidic pH below 7. Typical oscars purchased from pet stores will thrive in any pH between roughly 6 and 8. The most important thing to provide your fish is a consistent pH. This cannot be emphasised enough. A consistent (and stable) pH is much more important than an “ideal” one and the act of trying to create an “ideal” pH generally results in an unstable and inconsistent pH.
For significantly more detail on pH and it’s potential irrelevance, reference the following articles:
Myths of pH Shock
Realities of pH Shock
KH (Carbonate Hardness)
In order to maintain a consistent pH, you need to have some buffering capacity in your water. Simply stated, the buffer in your water is made up of dissolved compounds such as carbonates that resist changes in pH. This buffer is measured using a KH test kit. In general terms, if you have hard water, your KH is high. If you have soft water, your KH is low. As fish waste is broken down, acids are added to the water. The dissolved buffer counteracts the acids and keeps the pH steady. Without a significant buffer capacity, your pH will drop over time eventually causing a pH crash. Generally you want to maintain a KH value of 3 or 4 at a bare minimum. Unless your tap water is extremely soft or you’re using reverse osmosis water, you probably won’t have to check KH often or add any buffers.
GH (General Hardness)
Measures for GH are basically irrelevent to Oscars. Oscar owners have had long lived fish at all levels of general hardness. The one caviot to this statement is soft water (GH Below 8GH-German Degrees). It has been my experience that Oscars maintained in very soft water can experience difficulties in healing wounds, with those wounds subsequently developing secondary bacterial infections leading to bacterial pitting. For this reason, if your water is very soft, I would recommend dosing with a supplemental trace element product such as “SeaChem Fresh Trace”. This product can be used without impacting either pH, KH, or GH, but still provide the needed trace elements that may be not be in sufficient supply in soft water.
Ammonia is the primary form of fish waste in an aquarium. Most of this ammonia comes from fish respiration (Osmoregulation) but some also comes from fish urine. Oscars can produce up to 30% of their mass in ammonia each day, so ammonia levels can build up fast if your biological filter isn’t working properly. High ammonia levels will burn the gills of your fish and potentially cause respiratory problems for the duration of their life. A cycled tank should never have any noticeable amount of ammonia. If your test registers any ammonia, you should do a water change to reduce or eliminate the levels, then figure out why you had ammonia in the first place.
Ammonia toxicity is based upon the temperature and pH of the water. The higher the temperature and/or the higher the pH, the more toxic will be the ammonia. Use this chart to determine at what level ammonia is considered toxic under your specific tank conditions.
Bacteria in your biological filter break down ammonia into nitrites. Nitrites are very toxic to your fish, although not as toxic as ammonia. Under normal circumstances, a cycled tank should never have any noticeable amount of nitrites. If your test results indicate any nitrites, you should do a water change to reduce or eliminate the levels, then figure out why you had nitrites in the first place.
A different form of bacteria breaks down nitrites into nitrates. Long term exposure to high levels of nitrate can lead to health problems such as stunted growth and Hole in the head. It is best to keep nitrate levels below 20 ppm at all times in an Oscar tank. This will be easier to do if you keep your oscars in a larger than minimum tank. Nitrates will get out of control quicker if you keep an oscar in a 55 gallon tank vs. the same fish in a 75 gallon tank. This is simply because the nitrates are diluted more in a tank with more water volume.
If you are unable to keep nitrates below 20 ppm between water changes, check the nitrate concentration of your tap water. If it is at or near 20 ppm, you will need to find another source of water that is not nitrate-contaminated. Options include bottled water, distilled water, reverse-osmosis water, etc. If you use distilled or reverse osmosis water, which will have little or no buffering capacity, you will either need to mix it with your tap water or add some buffer to prevent pH crashes.
If your tap water does not have significant nitrate concentrations, and you are having difficulty keeping your water below 20 ppm nitrates, you should increase the size and/or frequency of your water changes. If this doesn’t help, you should reconsider the stocking levels of your tank or purchase a larger tank.