I was topping up my garden pond last night with tap water via my garden hose and unfortunately, forgot that I'd left it filling up. So came out this evening, pond overflowing with fresh water and crystal clear so presumeably existing water replaced with fresh water and some of my fish are now dead. The remaining 4 fish appear ok but they are at the top of the water sucking for air and are a bit docile. Is there any 'household' material which I can put in the pond tonight, to try to save my remaining fish from dying?