redearslider.com

I'm moving. My Slider is over a year old now, more like 1.5yrs.

What do I need to keep his aquarium outside? I'll need a cover of some sort and the filter. What else do I need to be sure to do?

Where outside? Is it in direct sunlight? Is it a glass tank? I would be very careful with overheating as well as cooling. I don't think a glass tank can hold temp. very well outside.

true, vear is right. Do not attempt to use a glass tank outdoors they are meant to be kept indoors and cannot take the temp changes very well but then again some people have successfully kept aquariums outside. Another issue would be overflowing when it rains and you would be scraping the algae off all the time. If you do plan on moving your turtle outdoors your best bet would be a stock tank or you can do a DIY pond it doesn't even have to be dug up you can just use some wood and pond liner. Make sure you secure the top to keep your turtle safe from potential predators. Both options cost way less than a glass tank. Anything outside would also need a bit of depth so your turtle can go at the bottom when its too hot.

Here is another issue I am having. Seriously considering an outdoor setup other than an aquarium. The water goes from clear to yellow within days. I have a Fluval U3 filter, I cleaned the media.. I think I want to go with a tub setup. Will I need a better filter for a tub setup?

The U3 and most internal filters are not great for larger turtles. Outdoor setups have their own types of problems, though would be cheaper/easier to maintain.

you need a pond filter, google "DIY pond filter" there are so many options that cost nothing compared to what you would spend on something indoors like a canister.