I agree, you seem to be underestimating the gibba. It should be fine to put it in the tank even while the peat is still there. And there is nothing wrong with the water being brown - it means tannins are in the water, which may inhibit algae. Utricularia graminifolia will not catch mosquito larvae even if the traps were large enough because it is an affixed aquatic, but most aquatic utrics, such as macrorhiza and inflata, should have large enough bladders. There are some variants of gibba with larger traps that may be able to trap them as well.