You wrote that you changed the fish's water. How much water was changed?
Have you tested the water for levels of ammonia, nitrites and nitrates and if so what were the exact results?
How many gallons of water does the fish's tank hold?
When you adjusted the ph what was it prior to the change and what is it now?
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Thanks for the information.
Just one more quick question.
How long has the tank been up and running with the fish in it?
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Thanks again for the information.
Before I begin I urge you to have the water tested for levels of ammonia, nitrites and nitrates. An easy and economical way to do this is by bringing a water sample to you local pet store and they will usually test for free. Test results should indicate ammonia and nitrites=-0- and nitrates 20ppm or lower.
Differing results would indicate a water quality problem which can be rectified by a series of 15% partial water changes done daily until the water tests normal.
The exisitance of ammonia and or nitrite in any amount can be extremely harmful and often lethal to fish. High levels of nitrates are also dangerous to fish.
Ok--you did well by testing the ph because this indicates a problem as a ph of 6.0 is too low for Oscars as it is too low for most tropical fish. An ideal ph range for Oscars is 6.8-7.2.
Your intentions were good in trying to raise the ph but ph is very often difficult to maintain at a higher level if the water source, the water you are using, is soft and by soft it is meant lacking in dissolved minerals. Hard water on the other hand has a high content of dissolved minerals.
Before I begin discussing ph I'd like to cover some ph basics so that we are on the same wave length.
Ph is the measurement of the amount of acidity/alkalinity in water. A ph of 7.0 is neutral so that is to say that a ph below 7 is acidic and a ph above 7.0 is alkaline.
Water hardness is the measurement used to describe the mineral content of water.
Hard water has a higher concentration of dissolved mineral content and soft water has a low concentration of dissolved mineral content.
Ok this is where it gets a little tricky. There's a relationship between ph and water hardness that many fishkeepers do not realize or understand. A majority of the time water that has a high(er) ph will also be hard water and this happens because the high mineral content (hard water) will buffer the water thus keeping its ph at the high level that it is at. So in essence if you have hard water it will be very difficult to successfully maintain an acidic ph for any length of time. You'll be able to lower the ph temporarily but the high mineral content will just bump it right back up.
Re: Soft water, the reverse of what I wrote in the prior paragraph holds true for soft water. Soft water is usually acidic (below 7.0) in ph and this is because of the low dissolved mineral content of the water. It's difficult to maintain an alkaline ph (above 7.0) when there is soft water which is what I believe your water source is.
Please note that the pH scale is logarithmic. To put it into mathmatical terms a ph level of 5 is 10 times more acidic than a ph of 6 and a ph of 4 is 100 times more acidic than a pH of 6 so you can see that what might appear to be a small rise/decline in ph is actually a very big one particularily for fish.
So applying this information to your situation, 1) the low ph is harming the fish and 2) the fluctuations in ph as a result of attempts made to change it are also stressing the fish.
The answer: the water needs to be buffered and the easiest way to do this is to add some crushed coral to the tank. Crushed coral is sold the same way as gravel is sold.
Just take out about 1/4-1/2 of the gravel you now have in the tank and substitute it with crushed coral. This will help maintain the ph when you add the ph adjuster to raise the ph.
Just remember to raise the ph gradually, very little each day so as to not shock the fish.
Also remember to try and have the water tested as mentioned earlier.
Listed below are links which will supply you with additional information concerning Oscar care.
Best wishes and please let me know if you have any questions.
http://www.aquariumguys.com/oscar-fish-care-article.html
http://www.oscarfishlover.com/index.php/Oscar-not-eating/My-Oscar-Fish-Will-Not-Eat-Food.html
YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING
You're most welcome and also remember that Oscars, because of their ferocious appetites and subsequent waste products, need frequent partial water changes in order to maintain good health. 25% partial water changes at least once a week are good.
Also as your water source is apparently a low ph and soft water it'd be a good idea to frequently test the ph.
Thanks for your accept and do check out the links provided in my prior post.
Best wishes.
Ric