About filter for your 3 gall tank sorry i don't like filters. So i am no advice here. I don't think the filter will make any diffirence in his health. How much water changes you do with a filter?
Also make sure you swish/rinse the filter media with every water change because it will gather a lot of gunk and can also make him sick.
Don't forget to remove carbon from the filer when you use that Immune bust.
i posted the link in the tread where you ask advice on fish immune booster and i ask veggiegirl to give advice on that. I dont know if you need to use it on regular basis . I also texted someone else for advice on that.
The catappa leaves is the same as indian almond leaves and a lot of people recommending it on this forum so its a good to have them . Just rinse before you use it.
It so difficult to distinguish the fungus from slime coat. But fungus usually white fuzzy cottony patches on the body. Slime coat is usually on the edges just like you saying he has. And slime more clear and slimy looking.
That stuff '' fish immune Boost '' saying its treats fungus so i guess continue but don't overdose. Lets see what people think about that.