I'd expect to move the tanks to top floors if flooding is quite a frequent occurrence although the unexpected is beyond your control. We once had a super typhoon hit our place last year unexpectedly. Fortunately, my area rarely gets any floods but the others were unfortunate. I saw horrifying photos of their tanks covered in muddy water and stories of their flowerhorns dead or escaping. The super typhoon is very rare in my area which is why I used the word "unexpected" to describe my experience. It was the first time rampaging waters hit my province due to the abnormal heavy rainfall amounting to a month in a day.