I wont go the chemical route. It will only solve your problem temporarily. Replace phosguard if it has turned dark. Water changes will help(no need for a 50%). Increase flow over the area, blow the rocks with a turkey baster to get rid of detritus on the rocks, do your regular husbandry like cleaning your filter socks, rinsing sponges etc , do not over feed. But please post a pic of your tank. It will be much easier to help you.
Better yet remove it... It's a detritus trap IMO...U could also replace ur sand bed
yes, it will start a mini cycle. Best is to remove a little at a time until you have no more of your old substrate left. It will give your critters, worms, pods etc chance to move over to the new sandbed.What does your current sand bed looks like? very coarse?
What substrate is it?
how old is the setup?