You say the house is about 25 years old?,, I would think the concrete tank was there to service the building of the house to start with, especially being a bricks and mortar house.
Using the concrete as a settling tank is not a bad way to help keep it clean, but it would be better if it recieved all the water from the roof as aside to "most" of it. You can just bet, the dirty side of the house gets into the house tank first,,,:(
I've lived with rain water for years, and never had a problem with stagnation issues, only leaking, and then contamination through that leaking,, (inground water tank very poorly constructed, too close to inground septic tank,,:eek:). When that was discovered, a new plastic tank went in,, and yuck,, doesn't matter what they say,,, nothing beats the taste of a good concrete tank, or even better, a corrigated iron tank,, :)
Theres also lots of different types of pre filters etc that you can get on the market now, but a couple that we have tried, haven't been much good. Thought it was me not fitting them in the right way, but even when plumbers have done it,, still no better.
I think probably the best way, is to have a very small, (100lt) tank just prior to your main tank to act as a rubbish trap. You can periodically clean it out to make sure its all good.
:( I miss the farm,,,
ahhhh,, the serenity,,,

