Sorry I am going to be long winded here, and I will not beat you up, stuff happens and as a newbie the mistakes your making are common. Sorry for the loss, I saw that updated after I posted my response
It looks like it is laying flat out on its side, it looks like it is going to die. Oscars are very sensitive to water parameters especially when they are young. My large pleco was adopted from my sister in law who had it in a 20G long tank for 2yrs without a single water change, when I got it it was about 5in long and living in 4 inches of black water, so in my experience plecos are very tough. The pleco is now over a foot long in my 90G tank. The reason I am telling you that story is a perfect example of how one fish can survive horrid conditions that will kill other fish. If it had something white hanging out of it that my be an internal parasite but I have no experience with constipated fish so I dont know if that is a symptom of that.
I will still stand behind your tank being sufficient for the current size of your fish as long as your being diligent with water changes, LARGE water changes properly treated with dechlorinator and temperature matched as close as you can get it.
Back in November and December of 2009 issues of Tropical Fish Hobbiest there was a 2 part article titled -- Time for a Change: A Mathematical Investigation of Water Changes. It gives alot of information on water changes and how much you should do rather than what is commonly recommended, you can try to Google it and see if anything comes up but I will summarize a couple key things I took from the article. 20% weekly water changes are not adequate regardless of stocking, larger weekly water changes of 50% or more should be the norm, it does go into the science behind this theory but I wont get into that. It also explains that water added straight from the tap has a high concentration of oxygen, and if not diffused before adding it or while adding it it can cause oxygen levels in fish to rise quickly basically giving fish the "bends". I use a python and diffuse the water by spraying it against the tank glass or driftwood while adding it, basically out gasing the high oxygen content. Another part of the article is the 5 gallon oscar tank experiment which I found to be very educational and confirms some of my theories, that water parameters effect health more than tank size. In this experiment the author keeps a large number of oscars in a large tank but essentially there is 5 gallons of water for each oscar. He does this by doing daily 100% water changes. When I was a kid just getting into fish i raised an oscar to adult size in a 10G tank, while I would never recommend that now as a novice we all make mistakes, you are taking the proper steps to help your fish and you wont find any better advice than that on these forums and current books/ magazines on the market. I would recommend reading some of the fish magazines out there, while we give information based on experience the people writing these articles are doing some of it solely for research so you will get more scientific information than just what has been passed down from member to member on these type of forums.
I personally do things different from everyone here and I have been for many years so it isnt going to change unless I experience an issue, I rinse my filters in tap water, I refill my canisters from the tap as well so I dont have to prime them after every cleaning. The bacteria live on every surface of the tank so even if there is enough chlorine to kill them off in the filter the tank will repopulate very quickly, I have heard that they replicate every 1/2 hour some others will say every 8 hours, I can tell you I have never had any issues with water quality that resulted from my maintenance requiem, I have had issue with my own negligence and lack of proper maintenance. Plus I would need several buckets of tank water to clean my canister filter cartridges, I use prefilled carbon floss bags in my HOB filters as a matter of convienece and change them out couple months when I notice the flow has decreased or the water is bypassing over it, I have had disasters from waiting until they are falling apart. I also clean the filter components with tap water. THIS IS WHAT WORKS FOR ME, everyone has their own theories and ways of doing things and what works for me may not work for you. Many theories on forums are based on this what everyone else says so it must be right were I want to see the science behind those recommendations.
Ok sorry this is so long winded and I hope I didnt overload you with information, just remember your trying to provide the best home possible for your fish but sometimes things dont go well so just stick with it, If the oscar doesnt make it try going with fish that are more appropriate for the size tank you have now.
Heres a link to a forum discussing the water change article i mentioned earlier in this post.
http://www.aka.org/aka/modules/newbb/viewtopic.php?post_id=4471&topic_id=1103
my life sucks right now im going to hope the fish survives but i hate seing it in pain like this he is doing circles on the bottom i guess thats a sign of pain... there quite fast too i feel so bad for the little guy i am doing 75% water changes everyday until i can get the test kit
