Hello Speedbirdone Did you keep any of the old filter medium or any of the old tank water? It seems to me that by changing the filter and cleaning the tank has gotten rid of all the bacteria needed to convert the fish waste products to nitrite. you have basically undone the whole tanks cycling before you added the fish. The murkiness is probably algae and waste, can be removed by doing small water changes and letting the tank re-cycle, if you can re add some of the old filter medium to the tank just to give it a helping hand
Is it a milky white looking murkiness? If so it's likely to be a bacterial bloom which will resolve itself within a few days.
If you didn't move your old filter media into the new filter and you have fish in the tank you need to test levels of ammonia and nitrite daily and take measures to keep them below 0.25ppm as you will be doing a fish-in cycle. This article explains in more detail what will need to be done to protect your fish during this.
OK it's a bacterial bloom as the tank is now cycling from scratch. The murkiness will clear in a few days, BUT the water will become increasingly toxic to the fish over the next few weeks until the tank has cycled.
You need to test for ammonia and nitrite every day and change enough water for fresh, dechlorinated and warmed water to ensure neither ammonia nor nitrite exceed 0.25ppm. To help detoxify the small amount of ammonia and nitrite still present (you need some to allow the cycle to happen) use Seachem Prime in a double dose sufficient to treat the whole tank volume at each water change. This may mean huge water changes every day for a few weeks. This article explains what to do.
The other alternative is to order some of this and put it in your filter. It will remove most of the ammonia and nitrite, leaving just enough to cycle the tank. Be sure to get this exact product, and to still test the levels daily as the polyfilter will become exhausted and need replacing until such time as the tank is fully cycled.