Your tank is too small for a pair of Ryukin Goldfish. These fancy Goldfish get very big and are very messy, they should get around 5" long excluding their fins. For a pair of fancies you need more like a minimum of 30 gallons not 5 gallons! They look very very cramped in that tiny tank.
I would get a new larger tank asap, if that's not possible, rehome the Goldfish and get a single Betta.
If you are going to upgrade you need to do 50% water changes twice a week - you should NEVER do 100% water changes in one go as this destroys your bacterial colony. And you should only be doing that 50% twice a week because Goldfish are so insanely messy! With just a Betta in that 5 gallon you would only need do do 20% once a week.
If the gravel syphon is too big, you need a smaller syphon, but in this case you really need a larger tank! A syphon should work on an angle anyway as long as it's fully submerged and you should not need to remove the fish - again this is going to be difficult in such a small tank with large fish!
Your current set up is a bit of a lose-lose situation.