"...it won't help anything " and I'll add that adding salt to the soil can be damaging. And please....do not use any pesticide until you have carefully read and understood all of the directions. Also, home remedies shouldn't be used unless there is a good understanding of how they work, possible problems, and potential damage. Repeating jean again....don't use anything unless you know just what the problem is.

The over-whelming % of necrotic leaf tips/margins is caused by over-watering. A high level of mineral salts in the soil often accompanies the over-watering problem due to the fact that so many growers water in small sips to avoid soil saturation, thereby ensuring a build-up of salts. That you flushed the soil thoroughly is a positive for your plant. The salts on the outside of the pot aren't hurting the plant. If they were soluble to the point they were making their way back into the soil, they would disappear every time you water. They appear because water with dissolved solids moves from inside the pot to the outside through the wall. As the water evaporates, the dissolved solids are left behind on the outside of the pot. The next time you water, the process repeats, water moves from inside to outside and leaves dissolved solids behind on the outside of the pot. Since you've flushed the soil thoroughly, I would encourage you to supply a low dose of an appropriate fertilizer and henceforth keep a close eye on your watering schedule (to avoid over-watering). I'd hold off on repotting until June if you live in the northern hemisphere [you should include your zone & where you live in your user info, like my (tapla mid-MI z5b-6a)], as repotting now would not be in the plant's best interest. I'd even resist potting up at this time. The reason is, your plant is growing very slowly (again, if you live in the N Hemisphere) and will take a long time to colonize the additional soil provided in potting up. This increases the potential for over-watering, which is a more serious risk than the temporary impairment of growth imposed by root congestion, which doesn't appear to be an issue, though one can't tell by looking at the photo. Around the summer solstice (Jun 21), or a little earlier for this plant - depending on where you live, I'd do a full repot and make sure the plant is going into a medium you can water to beyond saturation (so you're flushing the soil whenever you water, w/o having to worry about the consequences of a soil that stays saturated for extended periods. Your plant will really appreciate a soil that requires you to water every 2-4 days. Al

Generally speaking - and I'm no expert here, just have a few houseplants and have been learning a lot from this forum - some plants such as the softer leaves ones will show their displeasure and happiness quite quickly by wilting or perking up quite quickly. But I think some of the thicker leaved firmer plants can look ok for some time even when declining (more water reserves, stronger leaf structure,etc), for example sansevieria (snake plants), so maybe rubber plants are like this too. What I'm trying to say is it may have been going downhill for longer than the two weeks of obvious symptoms. Plus it's possible really high numbers of fungus gnat larvae could have munched on the really fine feeder roots. Although I thought the roots looked good overall. I don't know about the white bugs, you'd need to be sure what they were to know if they damaged it. And the (necessary) repotting was another stress on the plant. Many factors to consider, playing plant detective. It may well be the additive effect of several things. You've had lots of good advice from the experienced people above, on light and soils etc. Personally I would now put it in a bright warm draft-free spot inside, and wait. Don't let the roots dry right out while they're recovering but don't let the soil stay damp either. To achieve this, try using a spray bottle of water on a gentle jet to target where the pruned rootball is, and at the times when you do water more thoroughly leave the pot tilted at 45 degrees (even 30 degrees will help) for 15-20 minutes afterwards and then sit it on towels or paper pushed up against the drain holes for a couple hours. This isn't to remove all the water you just put in, it is to reduce the saturated layer that occurs at the bottom, the "perched water table" (yes, it has a name) Dont fertilise until you see new leaves growing and don't sunburn it by putting it in direct sunshine outside if it's not used to it. Good luck!

@Rebecca/N. IN/z6A thanks for your reply! :) Yes, I used soil with lots of perlite, bark and some other stone that I can't remember the. name of. The soil does drain fast and there's always little bit of water leaking down the bottom, but the plant sucks it back up. I never water first, but fertilise altogether with the water when the soil has dried out completely. I'll flush out the soil to make sure that there isn't too much fertiliser build up in the pot. Do you think there's anything else I could do about this? Thanks again!