Need some help with my Danios tail issues

I got 3 Longfin Danios from my LFS 5 days ago and put them in my quarantine tank. I noticed some white spots on their tails, figure it was ich and kept the temperature high and did a full course of treatment with API Super Ich. I have not seen much improvement, but I know ich can be hard to get rid of sometimes. Do you think this is something else?

Here is @Colin_T’s advice on treating ich:

“The safest treatment for white spot (Ichthyophthirius, aka ick or ich) is heat. Just raise the water temperature to 30C (86F) and keep it there for 2 weeks. Then reduce the temperature.

Wipe the inside of the glass down with a clean fish sponge.

Do a 75-80% water change and gravel clean the substrate before raising the temperature.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.

Increase aeration/ surface turbulence to maximise oxygen levels in the water.

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The following link has information about white spot. The first post on page 1 and second post on page 2 are worth a read.

(I also suggest getting at least 3 more danios)