I have a red oscar and a tiger oscar in a 65 gallon tank. The tiger oscar bullies the red one but not they also swim together and seem to get along mostly. Before I did a water change yesterday I noticed a white blemish on the red oscar's eye and he was slow moving etc...which is why I did a water change. The red oscar is worse and will not eat. Looks like the tiger oscar has picked at th red one a little more then usual. I changed about half the water and added about 2 capfuls of tap water conditioner. The red oscar is also gasping...breatging funny.The tiger oscar is fine. Can anybody help me out?? Should I try a another water change?

Before you start changing any more water, you really need to test your water. It's not good to change too much water, you will end up removing a lot of good bacteria. First things first, carry out an ammonia, nitrite, nitrates and pH test. Obviously ammonia and nitrite must be zero. Toxins in the water could possibly make a fish gasp. Only when nitrate levels are very very high will fish gasp. Since you've carried out some water changes, I expect you will find your nitrate is quite low.

Is the fish that is breathing heavily hanging at the surface at all, or is it something around normally and just breathing heavily? If you can take some photographs, we could have a look. We want to determine whether there is any problem with the one fish that is breathing heavily so water tests are very important

Thank you so much for your response. I was in a panic so took a chance and did a little water change. The fish started doing better right away. I took a look at my water conditioner bottle and discovered that I put 10ml of conditioner in yesterday when I did my water change instead of 2ml...that may have caused the issue?? I will have my water tested tomorrow just as a sanity check. Also, even though he was gasping and breathing heavily he was still swimming about. I did take pics but had trouble attaching them.

I have to disagree with OFL. There is very little bacteria actually in the water compared to whats in the filter.

Water conditioners are designed so that if you accidently overdose you wont harm your fish. I suspect your red oscar is just suffering from being bullied. You may have to separate them if it continues. However it may be your water quality so post the results when you get them. It is always a good idea to get your own test kit and test at least monthly but ideally weekly

Yes, you are correct, there isn't an awful lot of bacteria in the water. However, I think extremely large water changes should only be carried out when absolutely necessary. I'm always a little bit conscious about having freshwater flowing through my filtration. Okay, using a water conditioner will neutralise the chlorine, I just prefer to have properly cycled water running through the filters, rather than completely freshwater.

The other thing you have to do think about is pH. In many cases, a very mature aquarium will have a ph completely different to what might come out of your tap. My 125 gallon tank is a prime example of this. The water in it is 6.0, my tap water is 7.5. Doing extremely large water changes will upset the balance.

In a properly stocked aquarium, you should have to do no more than 20% water change a week, and this is purely to keep the nitrate levels down

I can see why doing large water changes are not a good idea in your 125 seeing as you have differing PH. The water where i come from is sooo hard it dont make a difference. I do 50% water changes every week

Even tho i keep discus im not much up on PH. All i know is my RO machine produces water with a low ph and hardly any impurities and i mix it with tap water to get the PH right. I used to have peat in the filter and that dragged to PH right down so it had to go. Also bogwood can reduce PH too. Do you know why your PH is so much lower than your tap water

Plants can increase pH, I haven't got any of those. Fish waste can lower pH as well as wood, so I think this is probably why my pH is so low.

My tap water ph is 7.2 and my tank stays at 7.2. I do 50% a week so this may explain why my ph dont drop