I don't know if I should be worrying but I have recently added a new fish to my tank, 12 days ago. The tank was set up in July of this year, it's 60 litres, with gravel and a (cheap) filter and plants. The ammonia level is normal and I don't have any other test kits as I live in Japan and can't find any, I will get some in the next month.
Over the past 5-6 days I've noticed algae growing on the tank glass. So today I bought a new filter and also cleaned the glass.
I also changed 10% of the water yesterday as that's when I noticed my fish was acting strangely. I usually change 10% of the water once a week.
My fish was darting across the tank and then twitching when it got to a corner, in extremely fast hyperactive movements, almost flipping over at times.
Since I changed the water the fish seems to be darting a lot less, but occasionally moves fast. But now I think I've noticed the top of her eyes are slightly bulging. She isn't acting her normal self.
I'm really hoping she doesn't get sick or die!!!!! by the way the other fish is only darting a little but looks fine.... Also they are both eating well, I feed them peas and pre-soaked fish food daily.
If anyone could give me advice I would really appreciate it as I dont' know what to do.
How often should I change the water at this time and how much??? should I do anything else?

Thank you

It's generally recommended that each goldfish has 10 gallons of water (roughly 73 litres I think?) all to itself in order to keep water quality at optimum levels and stop illnesses taking hold. With this in mind your tank is a bit overstocked so you need to be extra extra careful with your water quality. You really need to test for nitrites and nitrates as well as the ammonia. Incidently what was your ammonia level? It should be 0.Anything above 0 is harmful to your fish.
The darting is almost certainly caused by poor water quality. Until you get your test kits you will need to do 30% water changes daily to dilute any ammonia, nitrite or nitrate in the water and make things more comfortable for your fish. This should help a lot. Once your water paramters are normal (ammonia 0, nitrite 0, nitrate less than 20), you can go down to two partial changes of 30% per week and this should be enough to keep your water good. You also need to get yourself a gravel vacuum which will pull all the rotting waste out of your gravel and so improve your water quality. They aren't very expensice.
Is there a chance that you could be overfeeding too? Twice a day, as much as they can eat in 3 minutes maximum, but for now because you have water quality problems I would reduce that to once day as this will reduce the waste created and so keep your water better.

Thank you for the information, that was really helpful!
I'm getting a nitrite and nitrate kit posted to me tomorrow and my amonia level is 0. I only feed them once a day now and yesterday I bought a poo vacuum and I vacuumed the bottom of the tank today. The fish already look a lot better
thankyou for your help!!!