Well, there's some disagreement about this. Some people think it's the deep dark secret to perfect plants, and some think it's total hokum.

If you think about it, in nature plants are nourished by decaying organic matter. There are certain kinds of organic matter that humans generate which supply nutrients that plants appreciate.

Coffee and tea are also slightly acidic, and many houseplants (especially flowering ones, I think) enjoy a soil pH on the acidic side.

So instead of throwing out your leftover coffee, tea, pasta water, or the water you boiled veggies in, let it cool off and water your plants with it. (One exception--if you salt your pasta water, don't use that. Small amounts of milk and sugar in the coffee are fine.)

Maybe your plants will enjoy it and maybe they won't care, but I've never heard of anyone killing their plants this way and at least you'll save water. I've been doing it for a while, and it's certainly not hurting any of them.

You probably can't get away with doing this instead of fertilizing, because although your plants are bound to get some nutrients and minerals from your cold coffee, they won't get all of them.

Here is a link that might be useful: Feeding your plants with coffee, tea, pasta and vegetable water, etc.

What really works well is discard water from aquariums (if you have one) - all plants seem to appreciate it. But otherwise, when in doubt, don't - add odd things. If your plants need more acid, provide it with fertilizer.

I'm sure it works fine for many people - I've done it during a drought - but I prefer not to add anything with sugar or creamer, too likely to create mold and bug issues.

I had a sanseveria in an office that got nothing but what was left from my last cup of coffee for 13 years. It just set there doing nothing as office sanseveria are expected to do. Upon retirement I took it home, increased the light and water and it began to thrive and put off pups.

BTW there is a thread on coffee as a planting medium on the experiments forum. I've a ficus planted in coffee grounds but watered with tap water. It is eight years old and slow growing, perhaps a little skinny, it wouldn't sell well.

Someone I used to work with swore by coffee - but. then again, her cubicle plants were barely hanging on and mine were thriving.

The fishtank water idea is good, though, as long as the tank is established. Water from a "mature" tank (one that's been up and running with healthy fish for about two months) is rich in nitrates. (I recycle water from my big goldfish tank and two smaller betta tanks for my plants - when my mom doesn't claim the bucket for her garden first :). It's a great way to conserve water, especially with a softener - it's such a pain in the patootoie getting buckets from the bypass faucet sometimes...)

Now what is 20-20-20?

A balanced fertilizer.

MSales, until you know just what coffee, grounds or liquid, can do for your houseplants, it would be better to not administer it.
Don't feed a plant full strength coffee. Putting dried grounds around the base of the plant and when watered, it would give to the plant a certain amount of acidity.
The dried grounds can work much like 'diatomaceous earth' and causes injury to soil bearing pests.

Tea on the other hand is recommended to be re-made after enjoying the cup of, and applying some to the regular watering routine on an every 3 or 4 watering.

You enquire about 20/20/20. May I suggest you do further research by Google into "what is fertilizer", and learn about the use of on plants in general and houseplants in particular.