recently I noticed that 2 of my avocado plants were developing brown and dry tips at the leaves. I am wondering if this is due to hard water buildup since I use tap water for my plants. I heard that this could be fixed by squeezing some lime into the pot every month but I do not know if this is true. I water the plant every 6-8 days and it gets plenty of sun since it sits on my table which has a SE facing window right behind it.
Re-pot the avocado in as big a pot as you can manage. Avocado trees need a lot of "root space".
You may also have minerals in your tap water. Potted plants are susceptible to salt buildup, so once a month, place the pot in a sink or outside, then let water flow completely through the pot and drain out the bottom. This will help wash out any salt buildup.
But the main problem you face is that your avocado is in too small a pot.