yes
No. Box turtles are complete land turtles and spend most of their time on land or soaking in a shallow water dish. Yellow Belly Sliders need a pond or large aquarium with a basking spot and UVB bulb. The two turtles housing needs are completely different and should not be mixed, So, the answer to your question is, No. Yellow Bellied Sliders and Box Turtles can NOT be housed together.
All turtles need a filter in there tank or they will get sick from all the dirt.
these are to make the turtle look special and its also the shell. Turtles need shells to keep them protected.
Any normal filter will work. Chacha on!!
Yes. Yes. Yes. If the turtle is in a tank. Any tank, any size, you need a filter. Turtles are dirty and the only time you don't need a filter is if the turtle is in a LARGE pond and even then if there is more than one turtle I'd probably still have a filter.
Yes! A lot of them do just need to find a good sized pond
They probably are growing algae (yellow and green). You probably need more chlorine.
You can use water from the tap but you have to use Aqa Plus which takes out the chlorine.
Aquatic turtles need to live in water. How deep depends on the species. Turtles like painted turtles or slider turtles need water to be as deep as possible. Turtles like musk and snapping turtles need shallower water. They have to have land too, a turtle need to be able to completely dry off when ever they want to. Tortoises or box turtles need a shallow, easy to climb in, dish that is big enough for then to fit in so they can sit in it and drink when ever they want.
Fresh water, with a strong filter. Change water once every month.
You should have researched both species BEFORE you got them ! This is like putting someone who has never flown a plane, in the pilot seat and saying - fly it !