Are you talking in pond or around pond? There is so much available now! I have a 12v set of four spots on my pond. They are connected via a transformer and give of loads of light in the evening. You can even buy colour changing bulbs for them too. They came from B+Q i think and have an optional daylight sensor, but I used a switch/timer instead.
Have seen a fair bit of solar powered stuff online, think it really depends what you want and how much your willing to pay to run them.
Not sure on what was in the tank, but some younger fish do appear slime coated at this time of year. This is due to a virus that every fish has, but normally appears in spring, but disappears again as the water warms up. Its called carp pox if you want to look up some images on a search engine. Its non treatable, but just unsightly.
If you want to build a "koi pond" you could be getting in to an expensive arena. Really depends what you mean by koi pond? Crystal clear waters with a variety of fish can be achieved reasonably easily with 2nd hand pumps and filters sourced through sites like ebay. I don't have a bottom drain, but a pump anf filter that turns the volume of water over every two hours or so. I didn't want to invest in quality koi at ?100s per time.
I now have a mixed variety pond (with a couple of 10" koi), semi raised approx 12' x 25' x 3' deep, couple of pumps and a box filter. Managed this on a budget of around ?500.
Have already started enjoying feeding them in the lighter evenings.
Sorry can not see the pics you have posted. Have you looked up Carp Pox through a search engine?
This effects many younger fish and is unharmful, but also untreatable. All fish carry the virus and younger fish show it as what can only be described as drops of candle wax. It will normally disappear as spring kicks in.