I have been getting my new 50 gallon tank ready to put fish in next week. But this morning I emptied it out because I have decided on building a sand waterfall, and made a worrying discovery! I took some pictures to post for you to see and will let them speak for them selves, in the hope that you can perhaps tell me what it is and advise as to what actions I should take. What I am afraid of is that I am heading for a leak in my tank, so I am reluctant to start filling it again! (The picture showing the whole bottom of the tank shows ...to the right... a nasty white patch, but I found out that it is water that has seeped between the bottom of the tank and the table, and that is drying out now. )