Newbie need help 75gal with Fluval 404 filter..what do I put in the media

The reason the water isn't clear is because of the huge bioload in your ten. The tank cannot cycle and so the ammonia is sky-high. Basically, a 10 gallon is a death trap for two oscars.

A 75 gallon is a great home for one oscar, and congratulations on the upgrade! I don't personally know if people have had success keeping more than one in one, however. Keep in mind these fishes grow in excess of a foot in length.

IMO I would put two layers of mechanical filtration, one for catching larger particles and one for catching smaller, and then two layers of different biomedia, such a bioballs and then fine foam.

Good luck with your new tank, and welcome to AC!

The filter is used to clean the "larger" stuff out of the water and to "store" bacteria that turns ammonia into nitrites, nitrites into nitrates. Water changes & vacuuming get rid of the dead stuff & nitrates, which live plants can do to some extent as well (nitrates).

It doesn't really matter which choice of media as long as it has a large surface area to hold the bacteria and is rinsed in tank water then replaced from time to time (when it is falling apart), being careful to always keep some of the media so that you do not break the cycle in the tank.

Keep carbon on hand in case it is needed to remove meds after treating the tank but is not necessary day to day.

The filter already has coarse filter pads down the sides... in my 405 I run the bottom layer with floss in it, then 3 layers of bio media... trays about 1/2 full so as not to impede water flow. If I need to medicate I trade one layer of biomedia to carbon after to clear out the meds.

swooshing the established media isn't enough... take about half your established media from the old tank and put it into one of the lower baskets and let it stay for at least 2 week...(half if you are leaving live fish in the 10g, all if you are going to empty the 10g)... you can also just run the filter from the 10g on the 75g for a few weeks... that will transfer the different types of good bacteria to the filter.

I would recommend having tests for ammonia and nitrates. Once everything is established you won't need them, but until then knowing if you have either and doing water changes to keep them low might make the difference when it comes to keeping your fish healthy.

Sorry to say but a 75g is still to small for two oscars they will need at least 100g tank if you want to keep them both. I recently had two oscars in my 180g tank and had to remove one because they almost killed each other.
I would recommend filling the bottom tray of your filter with filter floss and the top three trays with bio media.

actually swooshing the filter pads or sponges from an existing filter may be enough.. actually better is to wring the floss or sponge out in to the new filter intake.

you will need to move the oscars over and if their old tank is empty why not just place the filter media from the old filter into the new one?

btw I concur a 75 will not be large enough to house 2 oscars.

I run Seachem Matrix in all of the baskets in my 405.

the oscars will become increasingly territorial as they age. even 100 gallons is not enough. you will need a 125 gallon (i.e six foot tank) to house two oscars. and thats ones that are only midly aggressive