i doubt the tank will be to bad, i would say the water might be abit stale. maybe just use it for gardens. i have found those plastic square tanks pretty good i believe they are 1000L so one or two of them are handy as you can pick them up for about $100 but just make sure what was in them prior as some people such as farmers kept poisons in them for when they did lots of spraying. 25 years old brick house should be ok. i dont know exactly what the land is like but if it is a bit hilly maybe just a check of the piers.some one said before about just running the gutter straight into the water tank that is already there, make sure the gutters are clean first:D. that is odd that the big tank is below the house i thought it was tradition to put them above so they could just gravity feed back down.in my experience the more tanks the better they are a good investment, personally i don't like these plastic ones the taste well yuck. we have owned both steel and concrete ones both were wuite good, only problem with the concrete ones is that they got cracks and even after getting patched up one of them still leaks about half way down.all the best with it.
papa smurf

