Try adding TLC Start Smart to speed up cycling.It works very well for me whenever I start up new tank and dump heaps of fish in it without having any issues.
Dwaft gourami is fine in a 50 ltrs tank. Bristlenose for suckerfish. Ottocinclus don't do much and you need a group of them. They are not as hardy too.
If it's only for 1 little goldfish you don't have to drive yourself crazy about the filtration. Any small internal filter will do or even an airstone in the tank provided you do at least 1 30percent waterchage a week.
Ron
White background are mostly used by discus breeders and keepers of pigeon base discus . Discus with pigeon base will have a cleaner body without much if any peppering.
Don't look like anything to worry too much about. Do more waterchange and it should come right.
Regarding the metro you can try the vet beside Animate on Porana rd.
If it's just the bd not eating just treat the bd in a hospital tank .
Treating with prazi is a good way to rid any fluke and some worms.
Do at least two treatment to make sure you get rid of it.
I normally don't turn on light during treatment.
It could be the ph swing at different time of the day that is annoying them.
Can you do a few ph test at different time of the day to see if there is any fluctuation.
One first thing in the morning, afternoon, before light out and one at night .
Any tank with the plastic frame on the base you are not suppose to put it on poly .
It has to sit on solid flat surface so you don't void the warranty .