a tank that is 4 x 2.5 x 3 should be large enough even for two adult oscars . . . I'd rather it was a bit wider, maybe 5ft, so I wouldn't say it's ideal, but we're probably talking about more than 200G here . . .
once we've confirmed the water is okay - - not just ammonia, nitrite and nitrate, but I'm also curious what the temperature is, since it's in the basement - - and if everything checks out, I'd consider adding dithers . . . large barbs, danios, leporinus, etc. . . . but not goldfish; too messy, and wrong temperature