hoo-boy...
1 tiger barb is bound to cause trouble to the others. They can be quite nippy if kept alone and not in a group to dither out any distractions (other fish). Tiger barbs should be kept in groups.
Platies and a swordtail, I'm surprised they sold one swordtail... but with Platies, I think they're ok... it's the others that are a problem.
The algae eater, what kind...? Pleco, siamese algae eater, chinese algae eater, otocynclus <sp?>...?
2 gourami's.. what kind, the amount of different types vary, it's important to know what they are...
the silver fish, is the line horizontallu (nose to tail) or vertically, top to bottom?
If they are going from nose to tail, are they "danios"?
The black fish is more than likely a rainbow shark.
A lot of these fish are ok in a 10 gallon but certainly not if kept together.... look these up and let me know if I am correct on the types.
(your sludge is definitely due to bio-waste material)