Unfortunately, removing the undergravel filter will prove to be a PITA. You will need to first remove all the fish/rocks(into a seperate container with a PH and heater) then either remove the substrate (do a search on 'bare bottom' to learn about a no-substrate tank) to a bucket and then pull out the undergravel filter...or try to remove the undergravel filter with the substrate still in. BE FOREWARNED the trapped diterus and muck that you will stir up will gross you out completely
I recommend using a canister filter with a MICRON filter inside to vacuum out the diterus. I use a Magnum HOT (hang on tank) for many cleaning chores and it does an awesome job. You will probably have to rinse the micron filter several time before you finish.
Yes, 10-20 times per hour turnover rate
Remember, the undergravel filter is only TRAPPING diterus...it is NOT exporting it out of the tank so it is not doing anything but creating a sewage area. The key is to EXPORT diterus out of the tank, not to hide it. There are other types of mechanical filtration you can use that will allow you to clean the filter media thus export the yucky stuff
Algae? Are you using ro/di water or tap? Also, once you get your nitrAtes undercontrol the algae will subside. Having a sump where you can grow good macro algae will also help as it will compete with the nusiance algae for nutrients.