If it has baby fish in it, it should be fine for the Koi, but you may want to test the water first, just to be sure. You can get kits to test chemical levels in water at pet shops and some places like Petsmart will test it for free. The pH level should be between 6.8 and 8.2. (7.5 is ideal) Let them fully recover, once they have, let them rest for a few months and build up their immune system, (about 3 months) and they introduce them to their new environment in the spring or summer. One thing you should know before putting them in the pond is, Koi grow rather quickly and get to be about 6 feet long or bigger, so the bigger the pond the better! They will rapidly out grow their environment, so keeping one in a tank long term is very impractical. Comets get to be about 8 inches long, but can get as big as a foot long. We have a pond with roughly 60 comets and 8 Koi. The comets are all about 8 inches and the Koi are about 3 feet long and we've had them for 11 years. They were about 5 inches long when we got them. Koi are a major commitment, as the oldest living Koi on record was 250 years old. They typically live to be about 35 or so, but in the right conditions can live well past hundred. Comets however usually only live for about 12 up to 15 years. With the size they are now, you could start with a small pond, but you will have to enlarge it every so often. You don't need a filter, but it's highly recommended, especially for smaller ponds because nitrates and other harmful substances can build up, killing the fish. Dead leaves and plant matter can be harmful if left up cleaned for long periods in a small pond, but if you have a waterfall and filter, some dead leaves can become good hiding places for babies and good hibernation spots for frogs or turtles. If you have a strong enough filter the water will be rather clear and you can easily see the fish, but you will need to feed them. Believe it or not, a basic comet goldfish pellet food and oatmeal (we use Quaker Old Fashioned Oats) is a perfect diet for them and can be kept is inexpensive. The pellets are perfect for the surface feeding goldfish because they float, and the oatmeal sinks, which appeals to the bottom feeding Koi. You have to be sure that the pond is deep enough so they can survive the winter. As long as you have a deep enough pond (3 feet or deeper) they should be fine. Just be sure to stop feeding them once the temperature of the pond gets below 50-55 degrees because it speeds up their metabolism, making it difficult to hibernate properly. Even young fish should be fin as long as they can hibernate properly. If you don't have a waterfall, make a hole in the ice daily with a hairdryer or something. (NEVER BREAK THE ICE! it sends harmful shockwaves down to the fish) The hole will let harmful gasses escape and let in oxygen. Waterfalls are very good for aeration, a big concern for Koi, and very attractive as well. Turtle and other creatures are fine to add with them, but as they get bigger they may eat any fish that is small enough to fit in their mouths. They will most likely spawn once you move them to their bigger home. They typically breed before it rains or on a warm, humid cloudy summer day. You may notice a little foam on the water's surface and see the males chase the female in attempt to get them to lay eggs. They lay a lot of eggs, however if you don't separate them or don't realize that they are spawning, most, if not all of the eggs will most likely be eaten. If there are plants in your pond, or small places for them to hide, some may survive and will come out when they are about that size. When ours spawned, we had 6 mature adults spawning so there were millions of eggs! They were everywhere! But they were all gone within a few hours. Of course our comets helped as well. Often when the two types of fish spawn they will make a Koi/Comet hybrid. Hybrids typically will present physical traits of both the goldfish and they Koi and can vary greatly in appearance and color, though usually the hybrids are very dull, usually black, brown or dark gray, sometimes with other colors present like orange on the belly that increases with age. However it isnt uncommon for hybrids to be the same color as the rest of your comets or Koi and could be very hard to distinguish. We had 4, 2 were pure black, one was black with and orange belly, and one was black with and slight purple iridescence. They grew to be about the size of most of out comets, slightly larger than ours, but still in the normal range, about 10". Hybrids are sterile and come from accidental cross fertilization, not from deliberate breeding. You said you recently added some plants to your pond? Any change in environment can stimulate the fish to breed, though one of the biggest is moving them to a bigger pond. Koi and comets typically won't over breed, meaning that if there are plenty of fish or their environment isn't very big for their size, they won't breed. Though, again, that's only typical and of course some exceptions may apply. Plants like water lilies and cattails are very good to put in your pond because they provide shade during the summer and release oxygen into the water, though again, if you have a small pond, be sure to remove dead plant matter when necessary. If you have a lined pond (like we do) you need to put them in pots, but be sure to put a lot of large roks at the top of the pot covering the roots because Koi love to dig in them and eat the roots. They will also nibble the leaves of the water lilies, but as long as you have a healthy plant it shouldn't harm it. Because it's fed by an aquifer it should stay at pretty consistent level, but you'll need to observe it in the summer to see how low the water gets. If it lowers more than a few inches, you'll need a system of adding water to it. It's best if you use a float valve so that it's self regulating. You may need to keep a net over it to keep out the birds, or strong fishing line strung over the pond, especially at the water's edge. But if you do get the snapping turtle that should keep out the heron. An authentic looking floating alligator head is great for deterring heron and raccoons as well. If you feed them regularly, they will actually grow accustom to you and come up to the surface when you walk by. We fed our animal crackers once, more out of curiosity and they took it from our hands. Before long they came to know the site and smell of our hands and no longer saw them as a threat. They actually liked it! They swam up against our hand (as though they wanted to pet!) and they kissed our fingers. You'll find that they can actually be very sweet, almost loving fish and are very good pets. I have the website we built shortly after constructing our pound. We only had WebTV at that point so it's rater rough and it hasn't been updated since. :/ WE have enlarged our pond greatly since then and our fish are MUCH bigger! But here it is if you want to check it out. And any other research is HIGHLY recommended! Most sites have a wealth of knowledge about Koi, comets and pond life. I hope that helped! :)