Fish Gasping For Air - Help Needed Urgent!

I have a 450L freshwater tank and it seems all of my fish are swimming at the top of the tank, gasping for air!
Things to point out are: 2 hours ago I had done the full test to my water to see what was going on and my ammonia in my tank has spiked to quite high (dark green colour on the test) everything else turning out fine, so I naturally done a 25% water change to dilute the ammonia in my tank but my fish have all gone back to the same way they were.
I did put in a couple cap fulls of API ammo lock to see if that would also calm down the ammonia in my tank but it seems nothing is working for me.
Someone please help me as I have some very big fish and do not want to lose my scales babies

 

I would do a much larger water change.

 

When your ammonia is spiking that high, a 25% water change is not sufficient to lower the ammonia to a safe level. Although the API product will help, you'll probably have to double the dosage and perform a 75% water change tonight if possible.
Tonight you'll also need to find out what is causing the ammonia problem. Is this a new tank that isn't yet cycled? Did one of your large fish die behind a rock somewhere? How many fish do you have in that 450L tank and what are their sizes? Is your filter working?

Your fish gasping for air at the surface is indicative of serious ammonia poisoning. The ammonia is burning their gills so they cannot extract oxygen from the water. They are suffocating.

Don't wait start a water change immediately.

 

I have started doing a much larger water change to lower the levels of ammonia.
I should point out the fact I did do a gravel clean and filter clean, along with a 25% water change all on monday. In doing so In my top filter i have running, there was 1 pad i had changed without rinsing in old tank water first. I think that may have resulted in my tanks water going toxic and causing nitrite and nitrate to sky rocket through my whole tank. So I done a 75% water change and all was well once again.
Until yesterday my water had gone foggy and all milky with my fish resulting back at the top of my tank

 

I am glad it is better Makes sure to keep a close eye on things.

 

It's good that you did the water changes but if your biological filter was heavily damaged then you don't have sufficient beneficial bacteria to process incoming ammonia and nitrite.
STOP FEEDING for at least the next 3 days and continue to perform large daily water changes while your bio filter reboots.
You may want to pick up a bottle of Seachem Stability which will help greatly during this process. Best of luck.