Ilya, welcome to Tropical Fish Keeping forum. Glad to have you with us.
I agree with squishylittlefishies except for the last line. A bacterial bloom (which is what this does sound like it is) is caused by too many nutrients, or more correctly, organics. There are a few reasons for organics.
First, organics are natural; all fish food entering the tank ends up as waste organics. So the more fish, or the larger the fish, or the more you feed the fish, the more organics will increase. We deal with these in several ways. Live plants help a lot. Also regular water changes, 1/3 to 1/2 the tank volume every week at minimum. And vacuuming into the substrate during these. Plus not overstocking the tank or overfeeding the fish.
Your first problem is the fish. A five-inch Oscar in a 29g is producing a lot of waste, not just solid that can likely be seen but liquid waste, ammonia and pheromones that cannot, and this is going to get worse. This fish can be expected to grow at the rate of an inch per month if healthy, up to a maximum of 18 inches, although 12 inches is more usual in aquaria. You will need a much larger tank; at least a 4-foot 75g tank just for this one fish.
At this point, because you are new to the forum, I'm going to mention the fish profiles section, under the second tab fro the left in the blue bar across the top of the page. This species is included, and you will find more info there. The name shades, Oscar, meaning you can click this to see the profile.
A test for nitrates would be helpful, as a rise in nitrates can often mean organics are increasing. But without even knowing this, I would do larger water changes, at least half the tank, and clean the substrate well each time. I assume you have one of the water changing devices that makes this easy.
No mention is made of feeding, but fish do not need that much food, especially a fairly sedate (non-active) fish in such a small (to the fish) tank.
To the salt mentioned, this is not likely to have any benefit at preventing disease, and it will irritate the fish to the point of death, so I wold stop adding it. You can read more on salt here: http://www.tropicalfishkeeping.com/f...quarium-97842/
Please do not add any more fish to this tank, they will only make this much worse. While the bacteria bloom in and of itself is not a problem for fish, the underlying cause may be, and here it is. As I said, a much larger tank must be ready for this fish, and soon. A potentially large fish living in cramped quarters will not develop properly, causing additional problems that will only get worse and shorten its lifespan.
If a 75g is not going to be possible for whatever reason, I would re-home this fish quickly, to another aquarist or a store.
By filtered water, do you mean through a tap filter, or bottled? Is there a reason you do whichever?