Don't change 50% of ur water at one shot.. you are going to mess up all your parameters big time and your fish wouldnt be able to adjust to it. Instead start with 10% twice a day and slowly increase it in 4-5 days time to 20% and so on. What you can do immediately is to add more airstones into the tank and improve you filtration capacity in terms of GPH and media. You should also start to find ways to re-house the fish that you have so that the overcrowding will not kill them off. Come on guys lets help do the more urgent thing which is to save the fish and not flame him for housing them inadequately.