Sounds like too much too soon, there is no immidiate quick fix without chemicals and you really don't want to go down that route, best option is water change every 2-3 days making sure you knock the gravel as you'll have lots of uneaten food and poop lying there, also check to make sure you have no other dead fish lying under something as this doesn't help
plus do you have fresh plants, just give any dead leaves a prune.
also never completely removing the filter media, this is where your good bacteria lives, if you need to replace, only replace part of it so your keeping some of the good in your tank
if you do have too many fish, be prepared for high levels and frequent water changes
What type of set up do you have and what fish??