Thank you for your question. It's good that you're thinking about the health of your indoor plants. When it comes to fertilizer spikes and fertilizing in general, it all depends on what kind of indoor plants you are growing and if those plants are actively growing (showing signs of new growth.) With the proper light, water, and humidity, indoor plants do a nice job of producing the food they need all on their own. They only need fertilizer as a supplement when they are actively growing. Fertilizer spikes release small amounts of fertilizer each time the plant is watered, regardless of whether or not they need it. Fertilizing indoor plants when they don’t require the additional nutrients can result in salt build up and root damage, which could lead to plant injury or even death to the plant.
There are many general purpose houseplant fertilizers available in either liquid or powder form on the market today. These fertilizers are easy to dilute which allow you to tailor your fertilization dosage to best suit your plants needs. These forms of fertilizers also make it easier for the plants to take in vital nutrients such as nitrogen. Our recommendation is to always mix the fertilizer at half the label recommended strength. The label application recommendations are typically for plants grown in optimum conditions.
Spikes would also not be a good option for house plants that prefer it on the much dryer side like cacti and other succulents. Fertilizer plant spikes would be a good option for outdoor container plants that require more consistent supplemental nutrients during their productive growing season.
For more useful houseplant tips, check out the entire publication, Caring For Houseplants in Northern Climates by Deborah L. Brown here: https://www.extension.umn.edu/garden/yard-garden/houseplants/caring-for-houseplants-in-northern-mn/
Here are some additional Extension links to helpful information regarding humidity and light requirements for houseplants: