Intro and Fantail Chasing = Finrot?

[alix]

Hi everyone!

I am a boomerang freshwater keeper who returned to the hobby after 25 years and after researching fish personalities aspire to be a fantastic fancy goldfish parent. ❤️

I had the good fortune of buying my starter 20 gallon long tank setup and two beautiful medium-sized fantails a month ago from Aquarium Co-op. I've felt pretty good about my learning curve on their little ecosystem thanks to the great content available these days (thank you!). 

However I'd appreciate some advice: For background, I think I've identified that Poke started to have fin rot due to a small piece missing three days ago, then some brown discoloration starting two days ago, and small jagged areas today (photo attached). I began active infection treatment with erythromycin yesterday but am more worried about identifying the cause since water quality and aquascape don't explain it so I was sensitive to recent fish aggression, and... 

I believe Poke must be a female due to rounder, wider, bulging shape (though I can't see a protrusion and s/he may have slight tuburcles) while Boba is a male with an elongated, thinner shape with no protrusion and recent tuburcles. Today Boba has begun exhibiting chasing behavior, staying below and nudging at Poke (but not nipping as far as I can see). 

Therefore my questions are:

1/ can you confirm chasing would explain why Poke has recent fin issues due to nudging or stress, hence it is correct to continue to treat for fin rot to lessen the damage? 

2/ should I add a second female to change the ratio to 1 male : 2 female to give Poke a break, even in their small 20 gallon starter tank (weekly water change stats has been nitrates 50, nitrite 10, gh 150, kh 80, pH 6.4, cl2 0, ammonia 0)? I've attached a photo of their relative size to the tank and current plantscape. 

3/ I have decided on 75 gallons as their forever home; would increasing the tank size help even if I can't find the perfect second female quite yet (finding a stand I like is also proving to be pretty difficult)? 

4/ am I missing anything or even overthinking it altogether? 

Thank you in advance for your time!

Alix

Edited March 11, 2022 by alix

[StanF]

Hi @alix.  Welcome to the wonderful world of fancy goldfish!  Your tank looks really nice.

I looked at the picture of the fish and I didn't see any signs of fin rot.  It may be just starting and hard to see in the pic.  Personally, I treat fin rot, ich, fungal, etc. (pretty much anything "external") with cheap, simple, effective aquarium salt.  I would raise the salt level to 1/2 tablespoon per gallon and maintain it there for a week or two to see if it resolves the fit rot.  I've gone as high as 3/4 - 1 tablespoon per gallon to resolve ich and a white fungus on my gancy goldfish.   Aquarium Coop has a great video on how to safely use salt.  I highly recommend watching it and understanding the proper way to adjust the salt levels.

It is quite possible that one of the fish is "nipping" on the fins of the other.  You mention a missing piece from Poke's tail.  Salt would help heal this also and helps their slime coat fend off diseases.

I would not recommend adding more fish to your tank.  Technically, some people might consider your tank already overstocked with 2 fancy goldfish in a 20 gal tank.  It can be done with a good water change schedule.  I have two fancies in a 20 gal tall bare bottom tank.  I do three or four 25% water changes a week to keep my nitrates below 50ppm.  Once you move to a 75 gallon tank, you would be able to add a third fish.  The larger size tanks are easier to maintain the water quality and stability.

StanF