If you take a sample of your source water to a fish store, they should be able to test the KH and GH for you (call ahead to check first, just in case!) make sure to write down the numbers, and let us know. They might say that it's "fine" but fine doesn't mean anything really, need the numbers to learn anything, and you don't need your own test kit for GH and KH hen the numbers are usually consistent if you use one water source. While going to have the water tested though, also worth taking a sample of the tank water and having *them* test for ammonia, nitrite, nitrate and pH. No harm in getting their results along with your own tests.
The leaf is highly unlikely to be causing any problems, almond leaves actually have antibacterial and antifungal properties that are beneficial for fish health. If you're concerned though, or there is question about the water quality, then remove it by all means.
More concerning is the clamped fins. Clamped fins are a sign the fish is stressed, but the cause of that stress could be almost anything. Poor water quality, battling an illness, or even being bullied by another fish. Means the fish is feeling stressed, and that there's something wrong, but you need a LOT more info to pin down what that thing might be. Don't focus solely on the almond leaves or water as potential causes, when it could be any number of things. Personally I'd make a new post in the fish emergencies section about his clamped fins, and copy paste and fill out the template below so people can try to help figure out what the cause may be. We cannot see the fish or test the water ourselves, so the more info you can provide, the better the chances that someone will be able to work out what's affecting your fish.
You may cut and paste the template below and submit in your post:
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Tank size:
tank age:
pH:
ammonia:
nitrite:
nitrate:
kH:
gH:
tank temp:
Fish Symptoms (include full description including lesion, color, location, fish behavior):
Volume and Frequency of water changes:
Chemical Additives or Media in your tank:
Tank inhabitants:
Recent additions to your tank (living or decoration):
Exposure to chemicals:
Digital photo (include if possible):
