Yup, goldfish in the tank. I almost always have a small boatload of the cheapie goldfish in my horse tanks.
Didja know that if you put about 10 or so in a 100 gallon tank, they will keep the green ookie from growing on the sides and bottom of the tank? The little swimmers will keep your tank from looking like something from the Amazon or the Black Lagoon. The best part is you don't have to feed them. They have a smorgasbord right smack-dab in front of them all the time.
Actually, the green ookie (it's really algae but left to it's own devices will grow out of control) is good for your horse(s). It produces iodine. I don't know how and that's why google is your friend, so you can ask there.
I usually buy at least 10 fishies. Usually, about 5-7 will die quickly and the rest will thrive. It's sort of survival of the fittest or the strongest. I used to buy them at Walmart for about 10 cents. Now they are about 28 cents. The best place to buy is The Grange. I bought 20 for a 1000 gallon tank and do you know, not one of the boogers has died yet?? Oh, and they cost a whopping 8 cents apiece!! Bingo!!!
What you have to do is to buy them the last thing before you go to the barn. You have to tie the top shut so you don't slosh a goldie on the floorboard but it also limits the amount of oxygen for them to 'breathe'. So beat it to the barn as soon as you pay for them. Then when at the tank, set the 'balloon' in the water and carefully open the top. Let the water in the fish bag come/go to the same temp as the water in the tank. After about 30 minutes or so, gently tip the bag over and let the little varmints swim out.
If some are going to go belly-up, you'll see them the next morning. If you put them in too quickly and then dump them, you'll see the carcasses immediately. But, for some reason, in my case, not one has died!
Oh, and another thing, since it can be really hard to see the fish in the bottom of the tank, buy only the brightest gold ones you can. Or if you see some that are almost white. Then you can kind of judge where they are and just in case you lose a fish to a bird, you can still sort of keep count. The dark brownish ones are fine in a pinch, just harder to see. Actually, damn near impossible...except you might see something go squirting by, then you can assume it's the brown one(s).
They will grow if they survive. In my case, if they all survive and grow, I may have to thin the herd a bit but I was thinking of starting up my aquarium anyway. At 8 cents apiece, I think that's a pretty good deal!
As an aside, if your tank is smaller and it's easy to see the bottom of the tank, make sure your horse is A-okay with the fish in the water thing. At first, he might snort if he sees something finny swimming away quickly but he'll get over it pretty quick. If he has any sense, he'll know he needs water when he gets thirsty. I'm saying this because some horses don't appreciate your efforts but they will come to accept it. Don't panic on your part!
Now, get out there and ride!! :-)