OK, but to come back to the original point -- you need to get a handle on two things. And frankly while there are ways to handle nitrates other than water changes, your best long term solution is (b) find out why they are so high and do something about it, and (c) do water changes until then.
Notice I left out (a) Determine if this is really your problem, and that may need to come first. 80ppm is not going to kill your fish, but that's 80ppm 3 days after a water change, before that were they 800, or also 80? Finding out how long it takes to get back up high is important.
Let me elaborate a bit with an example, assume you did a 50% water change at that time, and 3 days later they were 80. If they had been 160 before the water change, it means in 3 days they didn't increase at all. If they had been 80 before the water change, it took them to 40, and they increased 40 in 3 days, which is a fair amount.
And as pointed out by someone else, check the tap water -- I was talking with a friend who found his own well water excessively high in phosphates, others have high nitrates. Make sure.
Excessive nitrates that are not originating in the added water are almost certainly coming from fish waste - too many fish, too much feeding of them, accumulated organics in the substrate or filter that are rotting, plants rotting, a dead fish that you didn't notice, etc. Fixing these is the only real key to stability -- you CAN try to keep up with nitrates with excessive water changes, or worse with chemical treatments, but the best and least expensive option is to find the cause and correct it.
One thing I'd suggest is start keeping a log and for a while (shouldn't be necessary long term), test the water regularly for nitrates (and occasionally nitrites and ammonia just in case), including the day of, and day after water changes. See if the math makes sense -- 50% water change should pull them down 50%.
Also... don't test within 24 hours of adding Prime, as it may throw off tests (for an amusing aspect of it, read the last lines of their FAQ, where they claim a beneficial effect but do not know how it works
FAQ Here. Generally with well water you shouldn't need prime (unless it's a municipal or other system adding chlorine to well water), and if you are needing prime without chlorine you have other issues.
But... to the original point -- absent indications of really high nitrates, you may need to look elsewhere for the cause of death. AND deal with high nitrates.