Problem

it depends on whats actually wrong. if its just ph swings, making sure you add your nutes first, then checking it and making sure its in the proper range will solve your issue shortly.
if it's root issues, you need to fix them, or they'll get worse and your plant will at the very least be sickly and puny, and will likely die.
if it's a money problem, the cheapest solution would be to go to the store and buy two or three bottles of 3% hydrogen peroxide, and add 20 ml per gallon to your water. that'll kill everything in your soil except your roots. it also dissolves dead root tissue and aerates the soil.
benes would probably be better in the long run, they not only out compete nearly any harmful organisms, they help break stuff down in your soil, keeping your roots healthy and your plant fed. but you have to buy them to begin with, and then it really helps if you get some earth worm castings (ewc) and make your own aact (aerobically aerated compost tea)..put some ewc in a sock or something like that, tie it closed, drop it in a 5 gallon bucket of water, and put a fish tank pump in there making bubbles. you can add other stuff, but ewc is the base of it all. if you add hydroguard at about 75% of the suggested rate, and use aact weekly, it should keep your roots nice and healthy, and take advantage of your already loaded soil.

How often should i repeat that? And i mix it to the water and normally watering the plant?

i run hempy buckets, and they're a pain in the ass to run a culture in, so i run them sterile. i use 29% h2o2, the initial dose is 4 ml per gallon, then i give it to them every other watering at 3 ml per gallon. thats plenty to keep them sterile, and it helps dissolve old root tissue and aerate the reservoir.
when i was running big pots, i used benes. i would run half strength hydroguard all the time, and make aact tea once a week. you have to use it while it's still fresh. i would make a 5 gallon bucket, give the plants a little ph'ed water, then each one would get a good dose of the tea. then about 3 days later when they needed water again i'd feed them regualar nutes.
i also used a myco powder like great white, just sprinkle a little on the soil before you water, and it keeps your herd topped off.