But if all you have is a 2inch oscar(???) in a 120G there shouldn't be a lot of waste or feeding until it grows up.
Different tanks. One 2" oscar by himself in a 55-gallon, one mature oscar by himself in a 120-gallon.
TDS of your tank water is 1000 or your source water? (Also what is the range of your TDS meter as a lot of the hobbyist ones can't go past 1000).
I believe the current model from HM Digital (mine is a bit older, but looks the same) would be the TDS-4, which reads 0-1000 at a resolution of 1 ppm, and 1000 to 9990 at a resolution of 10 ppm. I am a sad, awful chemist for not calibrating it, but I figure so long as I know my RO unit is giving me a certain concentration and the current concentration is much higher than that, I'm making progress anyway.
Tapwater this evening came out at 654 ppm, TDS from the under-the-sink RO came in at 50-something, the wall-mounted one outside has read as low as 24 ppm recently.
As for the ~1000ppm on the 55-gallon tank, I don't always monitor the TDS out of the tap. Back when I had a job involving a very large RO unit and a very large tank for a decent-sized hydroponics research greenhouse, we would routinely check our source water; normally it was 500 to 1000, although others had reported spikes as high as 1500. Phoenix metro gets its water from canals (CAP water from the Grand Canyon), subsurface water from wells around the city, and Salt River water from the impoundments there. It is sourced based on availability and demand, I suppose, by hydrologists with a lot more qualifications than I will ever have. So I doubt that ~650ppm tap water became ~1000ppm tank water from evaporation- probably just some hard water days on refills. That tank loses very little from evap. (SEE EDIT BELOW)
The list of "organics" is plentiful. There are amino acids of various types, proteins, glycoproteins, sugars, and others. Anything with carbon and hydrogen is an organic (except bicarbonate and carbonic acid).
Assuming that the many known and unknown "organics" are bad may be wrong. Many of these may be useful in providing energy sources for various life forms in our tanks (bacteria being just one example).
It has been my experience from tissue culture and bioreactors that carbohydrates quickly become food for fungi, slightly less swiftly for bacteria. Proteins- or at least some of the component amino acids- are quite good as plant nutrients, at least in axenic culture: see also Malmgren's work with Soluvit and Vamin, which cracked the question of how to get nitrogen to plants without giving them ammonium or nitrate.
EDIT: I realized why the ppms were so high on the 55-gallon tank. A month or two ago, I tried to move the pH with vinegar. The water here has ridiculous buffering to it, it didn't budge. I added... quite a bit.