The python and other similar water changers cost roughly $45-50 for the 25 foot long one and around $75 for the 50 foot one, however you can sometimes find them a little cheaper online. I really want one myself, seeing as I have a 30 gallon, a 20 gallon, a 5 gallon, and change it all with buckets and a siphon. You can also make one yourself if you want to go that route, you can google DIY python instructions and find out how. As for dechlorinatong with one, add enough dechlorinator to the tank to treat the whole tank (not just the volume you removed) before you start to add the water back in. Remember to turn your filter and heater off before you start to take water out to prevent damage to either, and wait five or ten minutes after all the water is back in the tank before you turn the filter back on. This is to be sure that the dechlorinator has had time to work so that the new water getting sucked into the filter doesn't have any chlorine left in it to kill off your beneficial bacteria. Thinking of the filter, have you upgraded it along with the tank? The better the filter, the better off you will be as far as water quality goes. For goldfish you want one that filters at least 350 gallons per hour (10x the volume of the tank) and with 7 goldfish in there, you might want to aim for 500 gallons per hour or so just tone safe. On my 30 gallon tank I use 2 filters, one rated for a 30-55 gallon tank and another rated for 15-20 gallons for a total of around 450-500 gph if I remember correctly. I have 3 green spotted pufferfish, and while they are smaller than goldfish now they are similarly messy fish. Like goldfish they tend to be a bit messy with their food and they output a lot of waste. With my two filters I don't need to change as much water as often as I would with a smaller filter. Over filtration is a very good thing, especially in an overstocked tank like yours.
Thinking of goldfish and their messy eating habits... Do you feed them blanched veggies along with their staple flake or pellet diet? They really love veggies and they are very good for them. And another thing - goldfish love eating aquarium plants, but if you can get some fast growing stem plants in there that would help remove some of your nitrates as well. Sorry for the long answer to a simple question, but I hope I helped!