Sam

The light point cannot be overstated. I doubled the light over a tank that had sulked for eight months and it filled in over the following ten weeks with no other changes. I had spent that time blaming substrate and humidity.

If the furniture layout genuinely cannot go back, a cheap LED bar over the plant on a timer for eight hours a day will hold it. Not as good as the window, but far better than a slow decline while the household argues about the sofa.

The smell test is a good one. Healthy bioactive smells like a forest floor after rain. If it smells sour or like a drain, you have anaerobic pockets in the substrate and that is a real problem worth fixing.

Mealybugs, and you are killing adults while the eggs hatch behind you. Alcohol works well on contact and does essentially nothing to eggs, so a single pass is always followed by a fresh crop about a week later. That is the loop you are stuck in.

What clears them is repetition on a schedule for longer than feels necessary. Treat every 5 to 7 days for at least 4 to 6 weeks, and keep going for two more treatments after you stop seeing anything at all.

Places people miss, roughly in the order I find them:

• deep in the leaf axils where the new leaf is still furled
• the underside of the petiole right where it joins the stem
• inside the moss pole, which is basically a hotel
• the top half inch of soil and the crown at soil level
• the underside of the pot rim and the lip of the saucer
• the drainage holes

Rotate what you use rather than relying on one thing. I alternate isopropyl on a bud for spot work with a full spray of insecticidal soap or horticultural oil that coats every surface including stems and pot. The full coverage spray is the step most people skip and it is the step that catches the crawlers you cannot see.

Agreed. Bare rooting sounds drastic but a monstera shrugs that off far better than it shrugs off solvent poured over its roots.

Cover the screen top. That is the whole problem and everything else is secondary.

A full screen lid with a heat lamp above it is a chimney. Warm moist air rises straight out and dry room air replaces it, which is exactly the curve you are describing. The drainage layer cannot compete with that, because a reservoir under three inches of substrate has almost no evaporative surface exposed to the air. It exists to stop the substrate going anaerobic, not to humidify the tank.

Cut a piece of glass, acrylic or even heavy duty foil to cover 60 to 80 percent of the screen, leaving a gap along the front and around the lamp. That alone usually takes a tank from your numbers to something like 70 to 80 percent through the day.

After that, if it still swings, deeper substrate and more live plants both buffer it. Plants transpiring is a genuinely significant humidity source in a planted tank.

Almost certainly not the isopods. They occupy different niches and coexist in every established tank I have kept, and they are not predators.

The detail in your post that matters is the heat lamp running longer. Springtails are extremely sensitive to the substrate drying at the surface, and a lamp on for more hours dries the top layer even if your hygrometer still reads fine. They do not die so much as move down and out of sight, which looks identical to a crash.

Test it before you buy replacements. Lay a piece of cork flat on the substrate in the dampest corner and put a slice of mushroom or cucumber under it. Come back in 48 hours and lift it. If they are there, your population is fine and you have a moisture and visibility problem, not a population problem.

The drain access point is worth double. I built two tanks without it and both eventually needed tearing down because the water at the bottom went bad and I had no way to get it out.

If you cannot find a pot two inches bigger, root pruning back into the same pot with fresh mix is completely valid. Take the outer inch off the rootball with a serrated knife, tease the circling roots loose, refill.