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Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Can you post your photos for me to see these lesions? To do so, you can use the paper clip icon above your text box. Or if you cannot see that on your phone/computer, just post them on any site (ie Flickr, Photobucket, Imgr etc) and paste the web link here for me follow.
Can you confirm that your pond water parameters (pH, nitrates, ammonia, nitrites) are all normal?
Now since the photo is a bit hazy and underwater, can you tell me if the lesions look slick or fuzzy at all?
Does the one closer to the eye look raised in 3d, as it does in the photo? (where the one further looks more ulcerative)
Any redness or inflamed blood vessel changes along the tail or fin?
While I await your reply, I do want to note thus far that the foam is likely protein loss related and could be due to stress, waste build up (which will be quicker in a smaller tank), but also could be related to bacterial/fungal proliferation due to the lesion or systemic issues. And I have to note that while the lesions are what we can see, I am concerned that this goes further then just skin lesions since he was bottom sitting and had appetite loss. So, while high ammonia would be one differential for black ulcerative type lesions, his bottom sitting makes us wary of systemic disease (especially since the nitrates were hopefully <20) being afoot for this lad.
I await your reply to those further questions
If you are able to do so, that would be grand. Otherwise, the 2nd photo there is a bit clearer and I can see the depression lines on the paler lesion. Both actually look like ammonia burns (a chronic and more acute one), though again do have a check for vasculitis signs because his bottom sitting/appetite loss do make me concerned that the skin lesions are a minor issue and a sepsis is lurking for this lad.
Best wishes and speak to you later,