The number one killer of plants: WATERING. Too much.
1) Leaves turn yellow and fall off
This is typically caused by over-watering: too much water relative to the light and air conditions the plant is in. It could be you're watering every 4 or 5 days - but if the plant is not getting enough light, even that is too frequent.
Reduce watering frequency immediately, and find a spot with brighter light.
PRO-TIP: check how much "blue sky" a plant can see.
- If there are large overhangs or balconies right above, those will drastically cut down the amount of blue sky a plant can see.
- If the plant is too far inside a room then also it will not see enough blue sky.
Continuing to over-water or not give the plant enough light will sure as sugar cause root rot in your plant.
2) Plant is looking peaky and not putting out new leaves
The leaves aren't dying exactly, but just looking a little sad. The plant is not growing. Most likely caused by too little light. Most issues with plants indoors can be boiled down to "not enough light" - so make sure your plant is getting more light.
PRO-TIP: "Iight=food" for plants. If a plant's not eating, it's not growing. So give it more food.
- A corollary: plants prefer to get adapted to one spot if the conditions are right for it. So find one spot in your home where your new sprog will be happy and avoid moving it indoors/outdoors every day, or from one spot to another.
3) Leaves are losing color / growing longer or lighter or thinner or growing lighter green leaves
This is called "etiolation" - and is most obvious in succulents that typically have colorful leaves but also in many haworthias and aloes. Again - it's a symptom of not enough light.
The plant is desperately searching for more light, so grows greener leaves, sometimes they're longer, or more spaced out on the stem (i.e. not making compact rosettes), or thinner/weaker. They're greener because they're making more chlorophyll to use the limited light they're getting.
In a succulent like campfire, for e.g., you'd see that the leaves are far apart from each other on the stem, and very green, rather than growing right out of each other and tinged with red.
In a golden pothos, similarly, you'd see the new leaves coming out are only green, or darker green, and not with the lovely golden variegation this type of money plant is known for.
PRO-TIP: If you really can't find a spot in your house with the ideal lighting conditions, grab an ordinary desk lamp with an LED bulb in it and place it so that the light is a few inches (3 or 4 inches) away from the plant. Choose a bulb that is at least 15-20W. LED bulbs produce far less heat for the same amount of light so are a better choice to avoid burning the plant. Keep the light on for at least 10 hours every day.
(Note: this pro-tip works for smaller desk sized plants. Larger plants will need larger solutions.)