Q.Avacado growth suspended
Started from seed, grown to about 10″ ht. under grow light in basement. Typical light sometimes 24 Hr, then off for 12. Avg. day room temp guessing 70s, but cooler at night. Have added some miracle grow dissolved in water. Growth has ceased and now edges of leaves (5) turning brown. Suggestions?

Your Avocado is likely in need of more light. Spindly or poor growth is an indication of not enough light.
If you can give the plant a good sunny location and use the grow light to add some additional light that would be best.
Here is a link with more care information.
https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm