Could it be the tank/filter is cycling again ?
Cloudy water is sometimes a bacteria bloom it looks like slight "milky" water and happens when tank/filter is cycling.

I would completely tear down the new tank: that is,
take out all the old water, take out any substrate and wash out repeatedly with a hose outside until it drains clear, dump the fish altogether (throw them in the garbage), dump the plants if any. Take down the filter and replace all the filtering media that is replaceable.
Wash out the tank outside, lay it on a piece of indoor outdoor carpet and hose it down and use a scrub brush on every inch and cranny of it. Drain and dry off. Then reposition it on its stand and let it set for a week to completely dry out. Then rebuild with substrate, fresh water, conditioner, reset up the filtration, let it set for a week with the filter running. Then if you want plants set in plants and run a week again AND then after all of this buy a few fish-not feeders-you may want to try another pet shop in case the store you got the fish has its stock contaminated by fungal infections. And practice limited feedings, just a pinch of food 3-5 times a day. If any is not eaten then skip the next feeding pinch. Your conditions sounds like a combination of both buying infected fish and overfeeding. Do the above and take your time. If you need a large enough scrap of indoor/outdoor carpet check with a local carpet store or Lowes or what have you for the right sized piece.

How old or new is your filter media? Aside from dying fish, it may be the result of "new tank syndrome". Do you have an Ammonia/Nitrite/Nitrate test kit?