Basically by cleaning the filter you've utterly destroyed all the useful bacteria in the filter sponges that help to remove ammonia created by your fishes waste production ( poo, wee ect )
You need the bacteria in the filter to keep your water stable. You also do not need to completely empty the tank and wash everything. This isn't like keeping a hamster . Bascially you will now have to do a fish-in cycle. Your filter sponges need to grow bacteria on them. The bacteria eat the fish waste and break it down, making it less harmful to the fish ( ammonia burns the gills for one thing ) . Build ups of Ammonia, Nitrite and Nitrate ( all caused by fish poo and wee ) are all big causes of diseases and ill health , and your filter bacteria need to be healthy in order to cope with the large mount of waste goldfish make. If you take out the filter and wash the sponges under the tap, you completely kill your supply of helpful bacteria. You should only need to wash one sponge at a time, and only once every month or two( or when the flow coming out of the filter slows down ) at the most. ONLY wash the sponge in a bucket of USED tank water .NEVER a tap.
You should now leave your filter alone. Don't take it out, don't wash it, and don't empty the tank each week.
Every week, instead of emptying the whole thing, take out half the water and replace it with Dechlorinated water ( you can buy bottles of dechlorinator in fish stores and fish sections of pet stores ) . As the name suggests, dechlorinator takes chlorine out of the water. It is very harmful to fish.
Use a gravel vac ( again, found in petstores ) to suck any big bits of poo off the bottom of the tank. You do not need to take the gravel/stones out and wash them ( by the way, multicoloured gravel is AWFUL for fishtanks, the paint on them leaches off over time , stick with your glass stones or natural gravel )
Do your 50% water changes every week, and also get yourself a liquid test kit. Nutrafin Minimaster do one for about £20, and many people on here use API brand kits. Testing the water means you can see if you haveany levels of harmful Ammonia, Nitrites and Nitrates in your water. If your Ammonia and Nitrite are not reading as 0, you must do a water change at once to remove them.
How big is your tank and how many goldfish do you have in it?
