This thread has a bunch of info about filters and media.
viewtopic.php?f=9&t=30145I'd encourage you to search the forum for posts about media, cleaning, water changes etc. There's a wealth of information on this forum that contains answers to all of your questions.
Here's the abridged version of what you're going to find:
1> The 406 is probably too small for 80 gallons of water with 2 turtles. Read the link I posted for why.
2> Bio media needs to be rinsed, never replaced. Mechanical media like sponges needs to be rinsed and only replaced every few years when they begin falling apart. Chemical media (carbon) needs to be changed monthly.
3> Water change frequency depends on your filter and your water parameters. With properly sized filtration you're only doing water changes to keep the nitrate levels in check. You need to measure your water with a test kit to find out how much and how often.
4> Don't do anything during the cycle process... any changes you make to the water just prolongs the cycle from completing.