They have controlled movements when they swim and they don't look sick yet. But they were fine this afternoon- they were being playful and swimming around a lot this afternoon, but suddenly, they are gulping for air at the top of the tank, and I don't understand why. They do this excessively. I cleaned out 25% of the water immediately, and put in clean, de-chlorinated water. I took out a plant that seemed to dirty up the water (I think the roots were dying), and they are still doing it! What do I do to save my fish before the condition becomes serious? I love my fish. I don't want to hurt them. :0(
When fish gasp at the air like this, it means that they aren't getting enough oxygen from the water. Do you have an air pump or a powerhead? If not, I'd invest in one (power head if possible) and get it set up asap. If you do have one, it might have broken, so I'd double check that it's still working and replace it if you think it might be damaged. This will help oxygenate the water so the fish don't have to gulp at the surface for their O2.
It's true that your fish are gulping air at the surface because they can't get enough air, but they're doing this because there's something wrong with the water and it is unable to contain the amount of oxygen that they need. Definitely an airstone or increasing the surface movement of the water will help them feel better temporarily but that's just to buy you time while you figure out what the problem is. If you heat the water, you can also reduce the temperature a few degrees; the colder the water, the more oxygen it can hold.
I would continue to do large water changes, up to 50% per day, at least until you know what's wrong. Make sure you do take out anything that could be fouling the water, such as the plant you've already removed. If you haven't had your tank long, or you recently added a lot of fish at once, it could be that it hasn't cycled yet and toxins have built up to the level that the fish can't handle any longer. Check your water for ammonia - if that's high, add some ammo-lock. That won't remove the ammonia and you'll still see it on your test results, but it stops it from hurting the fish.
If you don't have an ammonia test kit, take a water sample to your pet store. They can test for other problems as well and let you know what's wrong with your water and what you can do to correct it.
I would continue to do large water changes, up to 50% per day, at least until you know what's wrong. Make sure you do take out anything that could be fouling the water, such as the plant you've already removed. If you haven't had your tank long, or you recently added a lot of fish at once, it could be that it hasn't cycled yet and toxins have built up to the level that the fish can't handle any longer. Check your water for ammonia - if that's high, add some ammo-lock. That won't remove the ammonia and you'll still see it on your test results, but it stops it from hurting the fish.
If you don't have an ammonia test kit, take a water sample to your pet store. They can test for other problems as well and let you know what's wrong with your water and what you can do to correct it.