fitting a polysterene base

[lornajane89]

Hi guys ive got a planted 30g setup which has been cycled since the start of november. At the minute the base is lined with cardboard which i know wont last all too long. Ive just found some perfect sized polysterene id like to replace it with. To lift it id need to empty alot of water out as i have noone to help except my heavily pregnant better half. Will i be ok doing such a big water change. Im also concerned my butterfly fish may eat my kuhlis while the water level is low and he has easy access to them. What are your thoughts and suggestions

Thanks