Why is this happening? Drying and Insects

Hello
Indoor and it's four months old and two feet high.
There are some very little white and black flies around my plant and also some kind of spider! too. White flies as shown in the picture like to go beneath the leave and hatch there!
And sadly leaves are drying too, so I just cut off some leaves from the bottom for the plant to grow.
Another one next to it is just fine and no insect dares to go around it but its leaves are going yellow.
Is it ok spray some insect killer around the bush? What do you suggest about the other one?

Read up on spider mites. Never had them myself so I can't be of much help.

looks like whiteflies, not mites

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html

Looks like thrips to me from the leaf damage. Spinosad will take care of them. For mites there are quite a few products. Do you have any webs?

Fighting pests sucks. Get on it asap or they will take over and kill ur plant.

Yes, there's also on the sick plant some webs.
I read somewhere spraying alcohol will effectively remove the mites and also soap water.
What if I use any kind of insect killer sprays?

Soak you entire grow with spinosad. Monterrey Garden Insect killer has spinosad in it. Start with that and see if it takes care of all your pest problems. Soak your entire grow: medium , plants,walls, cracks and clean everything really well before you spray. Clean thoroughly, spray. Day 2, spray again. Read directions on product, use when lights off.

Everyone, thank you.
They're mites. I bought a kind of insect spray and it worked, it was not spinosad though.

Keep applying every third or fourth day for a few weeks.