how do you fix the ammonia, nitrate, nitrite, and ph levels in a fish tank?

For nitrate/ammonia/nitrite (and probably ph) just do a water replacement. Do a 50% water replacement a couple times until they're in safe number if it's deadly high. If it's just higher than you'd like (such as it's causing too much algae growth) then do 25% water replacement once a day until you're back to the level you'd like 5ppms is a fine level. If this is a reoccurring thing with the ammonia then you're feeding them too much or putting too much nutrients into the water at one time. If it's nitrite same. If it's nitrate then your not filtering the water correctly or not doing enough water replacements (every other week 25% should keep the levels in good range depending on amount of fish and how much you feed them). If you don't want to replace the water that much get some plants. Plants absorb nitrate as a nutrient (so dose algae but it once it has covered all the good spots it wont absorb anymore). For Ph (BTW THIS HAS ALL BEEN ABOUT FRESHWATER, saltwater, I've heard, is different in the sense of controlling parameters) I don't really know what to tell you. you must be putting an acid or base into the water or something. usually one doesn't have a problem with ph for freshwater (at least I haven't ever) if you do there's products that'll change Ph for you.

go to your local pet store and get a chemical kit. It should cost about 10 bucks and would take care of all of your problems

use mild bases and acids for control. Do not use 95% sulfuric acid