I just went back to the beginning of the thread, he gave you an established tank. Cycling is getting the tank ready for new inhabitants. Generally a fishless cycle is used, meaning, you introduce an ammonia source (usually a raw, unseasoned shrimp) and let it rot in your NEW tank, this will produce ammonia (VERY toxic to fish), ammonia will spike and nitrItes (also VERY toxic to fish) will start to rise, as they consume the ammonia. The next step will be a spike in nitrAtes, as they consume the nitrItes.
In your case, they tank looks cycled or at least at the end of a cycle, judging by the algae present and possibly the high nitrAtes (not as toxic to fish, but over time can be, especially at high levels.
Ok, now for PWCs. I find it best to premix your water. You want to get your new water as close to your main tank's parameters as possibly, mainly salinity, pH and temp.
This is how I premix my water:
Day 1-add ph/salt/heater/water
Day 2-check salinity/temp
Day 3-remove old SW
and replace with new SW
, clean equip and carry on with my day.
With your nitrAtes that high, I would look into doing about a 50% PWC
, this should cut your nitrAtes in half. Start a new batch of water and wait 3 days or so and repeat, until you get them below 20. I know, this sounds like a pain, but once you get over this hump, keep up with regular PWCs and you shouldn't have a problem. This was just your friend's carelessness and you can easily fix it and have a happy, healthy tank.