Thank you for the pics. Still a case of unwashed sand. You didn't buy caribsea sand right? Even if you did, packaging will state pre washed if it is.
Nothing to do with DOC. If it is DOC, with that kind of foam, you have a big big issue on hand.
Back to technical aspects, it is not wrong for bros to prescribe the way skimmer works. Downdraft skimmers work that way so do Aqua C skimmers. Only thing wrong here being prescribed is that the skimmers all have constricted nozzles to create a high pressure directed water jet to create foam. In your case, a piping will not create that much foam. It will cause foaming but not Sentosa foam party kind of look.
Go change out those water with NSW and do some serious mechanical filtration. Chances are your filter pads will turn very brown.
Anyway imo, ten prawns are decent for your tank size but do add more bacteria and leave tank to the elements to speed up cycling. Thats the correct way as it worked for me.