this may be one of the tougher ones then never know
putting water in it for a lil while to kind of let it soak may help a fair bit if it's tough
like pre soaking very dirty dishes before using the good ole' elbow grease
most likely cleanable, but if he used it for cichlids there may be many small scratches from both substrate and fallen rockwork look closely
that dirt may be hiding a lot of hidden damage