Kent RO — Water purifier with cockroach

I bought 2 Kent purifiers. One in my native Bhubaneswar and another in my Work place Kolkata. To my surprise, when I requested service engineer to service my Kent in Bhubaneswar found that small cockroaches were floating inside the tank. He suggested that to buy an external iron/sediment filter to stop cockroaches and we did that and forgot about that episode. Last month I visited to my work place Kolkata and due to lockdown we did not use the water purifier for almost a year. So we raised a complaint and requested to clean the filters and water tank as well. I again saw cockroaches in the water tank of my new KENT machine installed in Kolkata home. This is terrible as both machines are non transparent. Again today my son found a little cockroach in his glass of water while extracting from KENT machine.Iis it possible to help us to avoid cockroaches in the water tank.