You shouldnt have any fish in your tank when you are cycling.
We all make mistakes when first starting the hobby.
You think lfs are giving you good advice when most of the time there not.
All you can do is water changes and increase aeration.
I would definately take the shark back to the lfs as he to big for a 15 gallon tank.
I wouldn't add anymore fish as you are pushing it stocking wise.
I would also pick a fluval 2 filter up to cope with your stocking, it better to over filter a tank anyway.
plus the juwel powerheads are not great filters.
Not a uni student are you sound like my son lol.
Vinyl records are expensive, flash back to the 70's then.
Good Luck.

nope banging uk techno
Don't bang to loud don't want to upset the fish lol.

soz to hear that![]()