Use 1 cup unscented bleach for every 25 gal water. So just use 2 cups for your 38 gal tank. Flow it thru every line until you smell strong bleach odor at each faucet, toilet etc. Make sure the tank is completely full and let it sit for 24 Hours to kill it all. Flush the tank several times the next day to clear the bleach and dead algae. The 24 hours is pretty important to disinfect the tank. (I work in the water and wastewater industry)
An open jug of bleach loses it's strength over time. So don't use last years half empty jug of bleach.