Oscar not eating

tank size?
tank mates?
ammonia/nitrite/nitrate?
water change schedule - how much/how often?
do you feed other foods besides the sticks?
have you added any new fish to the tank recently?

In my experience, not eating can be a sign of either an internal bacterial or parasite infection.

oh boy.

huh.

well. . . . let's start with the cycling process.

i changed my mind. let's start with the fish and the tank. unless this fish is 14" long living in a 30g tank, your oscar is stunted. stunted basically means that due to external influences including the tank size, water conditions, and the fish's own hormones (which build up in the water), the fish is unable to grow properly accorfing to its natural specifications. stunting causes bad color, an unhealthy immune system, organ failure (due to organs being jammed together inside the fish) and a shortened lifespan.

that is the first of the problems.

i guess not understanding the cycling process would be the second one. while filters remove physical debris from the tanks (uneaten food, feces, etc.) it is actually bacteria which keep the water clean. fish produce ammonia, the same chemical in cat pee (and our pee) and cleaning products. just like the fumes from a neglected litter box can burn your eyes, the waste ammonia produced by fish will build up in the water and burn their skin and gills. repeated or long term exposure to ammonia can make them weak and susceptible to illnesses. just like with people, it also makes it hard to breathe.

bacteria live in our tanks, anywhere with a high water flow and high oxygen. generally, this describes the filter. they also live on decorations and the top layer of the gravel. these bacteria eat ammonia, and produce nitrite. now, nitrite is just as harmful as ammonia, but there is a second type of bacteria that eats nitrite and turns it into nitrAte. nitrate is not lethal to fish, and generally safe up to 40ppm or so. (ppm = parts per million). most people would recommend keeping nitrates at 20ppm as a maximum however. nitrates have to be removed with water changes. not just topping off the tank, but actually removing a good amount of water and replacing it with new fresh water. not only are nitrates removed, but so are TDS - total dissolved solids. this is basically all the stuff you can't easily test for, unlike ammonia, nitrite, and nitrate tests which are available at any LFS (local fish store). it includes metals, minerals, hormones. . . anything that is in the water and does not evaporate out.

think of a glass of water, with a spoonful of salt in it. well, if you pour out half the glass every week and refill it with more salt water at the same ratio, the salt content (TDS) is going to basically stay the same. if you let the water evaporate out and then add more salt water, it is going to have more and more salt in it because none is being removed. this buildup over time becomes quite harmful to the fish and is known as Old Tank Syndrome. to use another metaphor - a fish in a tank with OTS is like a starving dog. the dog needs food, but is so used to being starved that too much at once can cause more harm than good. the environment needs to be improved a little at a time so that the fish is not shocked by the sudden change, even though it is a change for the better.

i listed all that because by changing out the filter every month (where the bacteria live) your tank is losing its beneficial bacteria and going into a mini-cycle, which means that most of the ammonia your oscar creates does not get turned into the less-harmful nitrate. the repeated exposure is not good for the fish. instead of throwing the filter pads away every month, take a gallon or so of water out of the tank and swish the pads around in the old water to remove debris, then put them back. this will not only save the bacteria, but it will save you a lot of money. they only need to be changed when they are literally falling apart, and in that case, put a new one in for a week before throwing away the old one. that allows bacteria to move onto the new pad.

to improve water conditions, you need to start doing regular water changes. i do not know the extent of your disability, but there is a product called a Python which makes water changes super easy. it is a long hose that connects to the sink on one end and the other goes into the tank where you use it to clean the gravel and remove water. no carrying buckets. then you regill the tank through the hose right from the sink. all you have to do is make sure the temperatures are very close. with the $$ you save on not buying filter pads, this will pay for itself very soon. another important investment would be test kits (not paper strip tests) for ammonia, nitrite, and nitrate. this allows you to see if there are toxins in the water, and how much. a lot of stores will test your water for you, but make sure that they use liquid test kits and that they tell you exact numbers, not just "your water is good" or "your water is bad - buy this to make it good".

you want ammonia to be 0, nitrite to be 0, and nitrate to show up on the test, but be under 40ppm for now. to start with a waterchange routine, you need to go slow, about 10-15% a day for the first week or two, then 30% weekly or 50% every 10 days. start there and adjust as needed to keep nitrates at a safe level.

honestly, one oscar should be kept in a 75g tank. while a 55g might be enough water for one, it doesn't provide the room to turn around freely - an adult oscar should be 14 or so inches, while a 55g tank is only 12" wide. i am very very surprised that your fish has lasted this long. i am not trying to be harsh as this whole thing was obviously kinda thrown onto you and you didn't know any better. it's just a shock that there haven't been problems sooner.

Hi there! Coming here to aquariacentral is the best place in my opinion to get advice for fishkeeping. It's a good idea to look through the freshwater forums for answers to other people's questions that you may not have even thought about yet. I've learned a lot here.
In regards to your question about changing the filter pad... you don't need to buy new filter pads unless the one you are using has begun to fall apart. You do however need to shake/rinse it out in old tank water (what you siphon out during a water change) to remove excess debris.
I wouldn't worry about buying water for water changes. I would recommend using prime to remove the chlorine from your tap before doing a water change. Also I recommend getting the Aquarium Phramaceuticals Freshwater Master Test Kit. I am assuming you probably have really high nitrates and should at first do 10-15% water changes daily for the first week, 25% water changes daily for the second week, followed up with a 50% water change. If your nitrates are still high (above 20) then another 50% water change. From then on I would recommend a 30-50% water change weekly considering the size of the fish and tank. Also it is a good idea to test your tap water for ammonia/nitrites/nitrates if you decide to switch over to using tap water. If you have any of these in your tap let us know....