spring cleaning of our water tank

[water guy]

Our gravity fed summer use system (on a permanent BWA) utilizes untreated water from a creek that is stored in a 1,300 gallon poly storage tank. We will soon be doing our annual cleaning of the tank and we're wondering if there is a best practice for doing so. After removing sand and sludge from the tank we normally utilize a dishwashing detergent to scrub the walls to remove the grime and algae and then flush the tank with fresh water, with the soapy water from the tank being spilled onto the ground. Several questions come to mind:
1) is their a biodegradable detergent that we would should be using in lieu of Sunlight detergent?
2) After flushing the tank would it be useful to pour a weak bleach solution into the tank (1 gallon of bleach to 1300 gallons of water) and allow the water to stand in the tank for 3 or 4 hours and then spill the water into the ground?
3) Could we also flush the distribution system using the bleach solution from the tank and then flush everything with fresh water?

[Islandguy]

Do you have to enter the tank to clean it? If so you should be practicing a confined space entry with a gas detector. When we clean we use a bleach solution and then suck it up with a vacum cleaner or sucker truck depending on the size of the tank. Use some bleach when filling the tank and get some into the distribution lines as well. Let it sit for a few days if possible, if its a weak solution to start with there will be very little residue left after a day or so. no need to flush everything out with fresh water