Ur plec is the common plec right? (Not the Zebra Plec or one of the miniature varieties rite?). Common plecos ate full size at about 15-18inches!!! Is yours REALLY full sized? I've got 3 plecs in my pond... at that sort of size.
That's my biggest problem with ur set up.

Next your bala shark.... err, this fella may be peaceful, but it also gets to about a foot!

Spiny eel.... It would be good if you checked the diet of these eels. They'd probably have ur danios for lunch and barb for dinner as it gets bigger.

For a 20gal tank.... u're WAY over the limit, with too many tank busters. For ur pleco, bala shark and spiny eel.... a 100gal would be more like it.