It looks like your doing just about everything wrong at the moment
What you need to to is to take control of the water
You do this by not over feeding regular filter maintenance and regular water changes plus by buying the following test kits Ammonia Nitrite Nitrate Ph Kh Gh, you then test on a regular basis
You can buy zeolite stones to take up the ammonia and crushed oyster shellto help with the Ph of your water , the rest is down to you and in self educating yourself on our hobby by reading .
The book I suggest is the Interpet Manual of Koi Health ..ISBN 1842860992 by Authors Keith Holmes and Tony Pitham, be aware there is now a revised version of this book out which we have yet to buy ourselves .
The quickest and easiest way of getting a coply would be on Amazon US cost as little as $4.00 used or $21.00 new :-
http://www.amazon.com/Manual-Koi-He...kmr0_1_cm_cr_acr_txt?ie=UTF8&showViewpoints=1
I promise you if you read this book cover to cover and learn well from it you'll be able to enjoy your fish for the first time .
By the way do you leave your waterfalls on in the winter ? Dont it can super chill your water and kill your koi.
Did you clean all the dead leaves and detritus out of your pond prior to winter ? Do it it all helps your koi to survive.
Did you clean and prime your filter prior to winter? Do it all helps in your fight against the winter period .
Do you do water changes throughout the winter ? We do using the trickle method which allows the ambient temperature of the water warm the trickle coming in at thats about30-40% of the total water (as a novice I wouldnt recomend you do this until you have learnt how.
Do you remove chlorine from the water prior to it going into the pond ? Do this is a must if you value your koi you can get dechlorinator liquid from your local fish outlet or on line .
Do you over feed your fish ? Quite simply dont , it is always better to under feed than to over feed roughly the rule of thumb is as much as they can eat in 5 mins feed up to three times a day , say once in the morning once midday and once about 18:00 pm
If your going to buy in fish to add to your pond? Dont not until you have built a smaller Quarentine Pond dedicated to QTing your fish for up to 2 months whist you check for signes of Parasites and infections etc and treat if necessary
Sound confusing dont worry the book covers everything you need to know , this year is our 27th year of koi keeping it mostly started out with two books for us and the koi manual was the second one
Dave
Dave