How To Clean A Fish Tank

The only pet I have is just one small goldfish. His mate died of old age. He was almost eight years old, which is a good old age for a small goldfish.
So now, I must buy another one to keep him company. In the meantime, I must clean the fish tank. It is quite a large aquarium for one fish. Anyway, this is how I clean the fish tank. Rightly or wrongly my fish have done pretty well so it has worked for me.
Before you start, find a large plastic bowl or bucket and make sure it is clean. I also keep a large plastic measuring jug, for emptying and refilling purposes. I switch electrics off: i.e. filter and light.
Using the plastic jug, I scoop the tank water into the bucket or bowl. I do have a fish net but I find it is less stressful for the fish if I scoop the fish into the jug and at the same time fill up the jug with water. Then very gently tip the fish and water into the bucket once it is almost full.
By now, the fish tank is about half-full or half-empty depending how optimistic or pessimistic you are.
Remove filter, light and any fish ornaments. Also, place any living plants in the temporary home.
Take a new and unused sponge scourer and start to clean off any algae around the inside of the tank.
As you work down the tank, keep scooping the water out and pour down the sink. Keep rinsing with clean cold water and once the water covering the gravel is clear, then use the jug to empty the water.
The most important things to remember are, to only clean with pure water, and never, ever , use soap, cleaning fluids or bleach of any kind.
In addition, always introduce some of the original water to the tank to keep the ph balance.
Next, dismantle the filter. Make sure you do not get the plug wet whilst cleaning.
Remove the sponge, rub, and rinse under running cold water until all the yucky stuff has gone. Put to one side. Pull out the torpedo shaped part that spins around and clean with simple rubbing and cold water. You can feel a magnetic pull as you remove this. Replace, when clean.
I have been cleaning these filters for years but if it is new to you please ask for professional advice because at the end of the day, it is electrical and you need to be safe. Next, clean any ornaments: i.e. shells etc.
Level the gravel and start filling the tank with jugs of cold water and a few jugs of boiling water to take off the chill. Fill to half way. Return the filter to the tank once you have put it back together. Plug in to the electric but do not turn on just yet.
When the tank is two-thirds full, fill the jug with some water from the temporary home of the fish and pour back into the fish tank. Gradually, add the original water from the bucket to the fish tank but leave enough for the fish.
Replace the plants and ornaments if any.
Check the fish is still okay in its makeshift home and make sure there is enough water for it to survive.
Turn the filter back on in the tank and let it run. Ideally, leave replacing the fish until the next day if possible. You can also buy conditioner for the water. Gently replace the fish and you will notice a very happy and curious fish swimming around.
Feed as soon as possible.
Of course, you will only take these measures when a full-scale tank clean is needed.
This is how I manage my goldfish but if you need information on marine or tropical fish or indeed any other detailed information you will need to ask the professionals. I always find this at my local Fish Aquatic centre.