I used a large hose. I ran the hose from the planter outside into the tank. Disconnected it from the tap (I have a quick connect valve so its easy to hook-up and remove). Used the output from the canister filter into the hose to start the siphon. Drained the water I wanted. Hooked the hose back up to the tap and fill the tank.
I am pretty sure you can find something on youtube. I don't have tanks that size to make a change of 300 gals on one tank. But I do have a 150/180/200 gal tanks. As far as pumping water outside I use Aquaclear 110 powerhead with a large hose. I keep saying i am going to make a video.
Are you talking about changing 300 gallons or changing water on a 300 gallon tank? Either way it would make me look for some easy way to get it done. I have several tanks and just those may me want an easy way so I did a little extra work today. I had been running all the old water down the drain but as it gets hotter and drier, I started feeling guilty. I now have a pipe through the wall to a part of the yard which doesn't get watered very well. I think that will be a better use for the 75 gallons I was dumping every 10-14 days. Every situation is different but I would look at putting in some dedicated plumbing for the water changes.
Either will do it. I thing most do it as the tank is filled. The small amount of chlorine in water is an irritant but not immediately serious. In a 300 gallon, I would add the required amount and then add the water but it does not seem to be critical. I feel keeping the temperature from swinging wildly is more important than when you treat the water added.
When doing large water changes, definitely treat the water first if the tank is stocked imo. Factors like pH are directly affected by hardness and temperature, something which varies depending where you live. RO water can swing above and below neutral pH. Buffer the kh, gh then pH. Use a large plastic tub to do bigger batches... a round garbage can with the separate wheel dolly works great (not the garbage cans with built in wheels, they leak).
Tap water is also ground-chilled - a bit too cold to go direct into the tank. By the time the water is treated and balanced, the temperature is close enough that the tank heater will be able to compensate faster.
I dose the tank before adding water as well. I run a python from our laundry sink so that the water can be heated if necessary (usually just in the winter). For removing water, I usually run a 25-50ft siphon hose out into the yard during the summer, or just drain back to the sink in the winter. For delicate fish or with tanks that have significantly different parameters than your tap water, you can run it into a large drum or trash can, adjust the parameters, and then pump it into your tank. I know a lot of people with delicate fish like discus do this.