Hi everyone,

I'm a new member to the forum but have been reading posts on here since last summer when I entered the exciting world of pond keeping! Tons of useful info so I'm hoping some of you much more experienced people can help me out.

We recently had a Koi death which was pretty sad. Actually we also lost five fish over winter (two goldies, two sarasa comets and a shubunkin). I was away for the entire period (it's at my parents house so they were in charge!) but apparently there was only one body found. We had a net over the pond so I'm not sure how the others could have vanished. Any ideas?

I recently read that most people turn off their filters over winter as otherwise the water temp drops too far. We left it running full time (it's an Oase Filtoclear 15000 and Aquamax Eco 12000 so the economy function slows it down in cold weather). Should we have stopped it? If so, do you need to clean it out/restart the bacteria when you turn it back on? Other than that I don't think we did anything wrong. We slowed feeding (using wheatgerm) during September/October when it started to get colder then stopped altogether around November time. The running water would have kept a break in the ice.

When I got back home and we uncovered the pond (April) there was a ton of blanketweed but the remaining fish seemed happy enough. We started feeding again about a month ago.

There was no easy way to get the blanketweed out so I did a full water change yesterday and took everything out (why did I put so much gravel in the bottom!). At the time one of my Koi (Goldilocks for obvious reasons) seemed very docile and was hanging around just under the surface. He almost let me stroke him which was uncharacteristic and we noticed his tail looked damaged. I had to move all the fish (and four newts!) into temporary accommodation (using water from the pond) while I emptied the pond and cleaned it.

When I was refilling the pond I checked on the fish and saw Goldilocks was lying on his side at the top. I thought he was probably on his way out so the stress of moving and different water temps could have finished him off. He was still breathing though so I quickly put him back in the pond. I know this probably wouldnÂt have helped but I couldnÂt think of anything else to do. I de-chlorinated the pond and added some biostart but didnÂt want to keep the rest of the fish in the "holding area" for any longer so dropped them all back in.

Goldilocks never perked up and I took him out this morning, definitely not breathing anymore. I could see any other external damage apart from the tail but have taken some photos for your opinions. Is it fin rot and if so, how should I avoid it reoccurring? I dosed the new water with "Tetra Pond MediFin" and also "Bacterad Anti-Ulcer, Finrot & Flukes" treatment. Overkill perhaps but is that a bad thing? Anything else I should have used instead?

Our pond is about 3000 litres (650 gallons) and has had the return pipe from...