I've lost citrus seedlings to sand knats before. Potted indoor plants are more susceptible to their knot larvae which eat the roots if soil moisture conditions aren't right. If you see knats flying around your pots especially when you water them, you got sand knats eating your roots. Also, spider mites are bad inside a house due to low humidity high temps, and the fact that you don't have high winds and natural rainfall knocking the mites off the plants. Ive had better luck growing seedlings outside when not freezing, in a semi shaded area to prevent the full sun from drying them out too much.