Since Kenny had to catch Rufus up. Rufus thought that Kenny was his friend. Rufus didn't come to Kenny's house because Kenny laughed at the mean comment Larry Dunn said.
Yes! The tense has to stay consistent throughout the sentence! ;)
By explaining problems and giving solutions! this is the best fit because the other two might make sense at a first glance, but only explaining problems and solutions would be an effective way to teach people how to set up an aquarium by giving them examples of what to do and what not to do whereas the others are just him talking about his aquarium.
