. To clear it up fast, an external filter hung on the back of the tank should do the job.. Now, why do you have a problem with "colloidal particles"?. You may be over feeding and there is an accumulation of food particles. If this is not taken care of, it will lead to ammonia and nitrogen problems.. Overcrowding is a common problem.. I have used undergravel filters with great success. A well-balanced tank with an undergravel filter requires very little maintenance.