If it's a new tank set up that you've just done, there really isn't any need to be doing water changes already, unless the sand wasn't rinsed enough, which i am a little worried about. Although it was supposedly already clean, it's always best to clean it thoroughly just in case.
The best thing you can do is leave it alone, it's just settling down at the moment, keep the filter and heater and anything else going continuously. If you have all the required sponges etc from the previous owner and you've already set the tank up, i wouldn't start swapping them over now, or the problem will just get worse.
You should have 2 fine sponges and a gap in the bottom basket and 2 coarse sponges and a carbon and a fine white one in the top basket, or you can do away with one of the coarse ones and put one of the new nitrate sponges in instead.
keep adding live bacteria, don't worry about the plants, they're not a problem. Add a TINY pinch of food every 5 days or so, to help start the bacteria off.
A tank that size will need to be running for at least two weeks, ideally longer, don't rush to put any fish in, it's not worth it. When you do start adding them do so very gradually, just 3 or 4 at a time and leave a week in between. Don't feed them much either and only do it every other day, so you don't over load the tank.
Just be patient and see what happens, it should start to settle soon.
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