Hard to find the answer listed but found this where Carlsson says it is mineral oil:
Carlsson: So what we plan on doing is to compare people who are on the therapy, so on the EPA fish oil compared to those who were on the placebo, which is just a mineral oil tablet, before and after 18 months of therapy.
Also, if successful we are looking at a longer trial:
Chin: So if successful, there will be future studies that will stem from this.
Carlsson: Yes. If this is successful, we'll aim to do larger scale clinical trials. It will be longer in duration.