The only way to get rid of the bubbles is get rid of the skimmer. You could try the sponge filter that should have come with it for the output of the skimmer. But in this hobby unfortunately you get what you pay for a skimmer that you can find for $14.50 is probably going to have its issues. https://www.fish-street.com/boyu_wg-308_protein_skimmer
Like Mr_X said earlier you may want to get rid of the skimmer and do good water changes. Just add a good power head get some surface agitation and you should by good. Try a Hydor Koralia Nano 425 or something similar.