Don't touch the filter media! lol The manufiacturers say to replace the media every x month(s) but don't do it. You can keep media until its literally falling apart which can take years. If/when you do need to change it out, just do a small portion at a time. You can add media at any time though.
What filter do you have and what size tank is it rated for? A good rule of thumb is to have at least double the filtration for your size tank.
For the nitrate test, I'd shake and bang BOTH bottles. For bottle #2 in particular you want to shake and bang it for at least 30 seconds, then shake (vigorously) the tube for a full 60 seconds then wait 5 minutes to take a reading.
I'm not a stocking expert but I think yours is doable. The denisoni's prefer cooler temps, but 78 might be ok for them. You're a bit heavy on the bottom though; the pleco, pandas, rams and eel are going to be at the bottom and if you get a male and female ram and they breed they're going to become a bit aggressive, plus they'll all be competing for food. Keep up with a good cleaning schedule; 50-60% weekly is advised along with regular substrate cleanings. Did you measure the tank? If I plug in a 58 gal tank into Aqadvisor I get different dimensions (36 x 18 x 21).
