The short answer
Plants need three things in balance: light, nutrients and carbon (CO2). If growth has stalled, one of those is the limiting factor. In practice the culprit is almost always too little light or, more often, missing nutrients — not a mysterious disease. Work through the three in order and growth returns.
Rule out the easy causes first
New plants routinely sit still for a few weeks while they adjust. That’s normal, not a fault. Before changing anything, check the basics: is the light on a timer for 6–8 hours a day, and is it actually bright enough for the plants you chose? A dim kit light will keep easy plants alive but sluggish, and won’t grow demanding ones at all. See how much light plants really need.
Feed the tank
A brand-new tank with plain gravel has almost nothing for plants to eat. Even easy species need nitrogen, potassium, iron and trace elements. The simplest fix is a weekly all-in-one liquid fertiliser; for root-feeders like swords and crypts, push root tabs into the substrate too. Our fertiliser picks and the fertiliser hub cover both.
When light and food aren’t enough
If you’re running strong light and dosing properly but demanding plants still crawl, the missing piece is carbon. Under bright light plants burn through the tiny amount of dissolved CO2 fast, and growth chokes. Injected CO2 transforms a high-light tank — see CO2 for beginners and the CO2 systems hub.
If you’d rather keep things simple, stick to low-light, no-CO2 plants like Java fern, Anubias and Cryptocoryne wendtii, and match your light and dosing to them rather than fighting the tank.