White fungus looking spot on my guppy

Hi! I’m new to this forum though ive been keeping fish (cardinals, guppies, ember tetras, pacific blue eyes, and a few other species) in a 3ft planted tank with no issues for a few years. I’ve just upgraded to a 5ft, 600litre (160gal) tank and im struggling to get it stable (temperature (too hot) and chemistry(GH was way too high at 12degrees, but I’m working on getting it down to 3degrees)).
All through this, I lost a few cardinals (I’m sure because of the GH), and now one of my guppies has a big white fungus looking thing on his pectoral fin (see pic).
Any help to identify what it is and how to treat it would be much appreciated .

This fish looks like a platy, not a guppy.

Please post your water parameters, Ammonia, Nitrates, Nitrites, pH. You are measuring GH in degrees, I assume it's in dGH numbers what you are posting. 7.0 - 8.0 dGH is good for platys.

Is this upgrade tank cycled?

Ram419 said:

This fish looks like a platy, not a guppy.

Please post your water parameters, Ammonia, Nitrates, Nitrites, pH. You are measuring GH in degrees, I assume it's in dGH numbers what you are posting. 7.0 - 8.0 dGH is good for platys.

Is this upgrade tank cycled?

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Great - I have a Platy A,N, N - 0/0/5. Ph 7.0 yes dGH - 8. Tank was Cycled by using old filter (plus new filter) for a week.

Ok, GH seems good then. I would try water changes and the salt treatment. https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/

@Colin_T might have better advice.

Ram419 said:

Ok, GH seems good then. I would try water changes and the salt treatment. https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/

@Colin_T might have better advice.

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Thanks mate!

The wound looks like an injury to me. So, not an illness. And yes, as already stated, it's a platy.

emeraldking said:

The wound looks like an injury to me. So, not an illness. And yes, as already stated, it's a platy.

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Thanks