As for the gravel. If you want to change it out I would suggest removing all your fish into a buvket filled with tank water. Then change out all the gravel. At this point do not change the filter media as well as it will be the prime source of good bacteria for your tank. In fact you rarely have to change the media in a filter. For sponges you just rinse them out when you do a water change using old tank water. Bio Wheels can just be rinsed off as well when the gunk builds up too much on them.
When changing out the gravel make sure to pay attention for ammonia or nitrite spikes for the next week or so as this will remove a lot of good bacteria as well.
When changing out the gravel make sure to pay attention for ammonia or nitrite spikes for the next week or so as this will remove a lot of good bacteria as well.