I'm sorry you are having this problem with mildew/mold.
A few thoughts:
1. It is unlikely people here can give you information that will eradicate your problem without inspecting your home in person (as qualified professionals).
2. No one here can bless your tanks and proclaim they aren't the culprit, or at least contributing to the problem. Even if someone says otherwise, information exchanged on an internet forum will not be contributory if you get into a dispute with your landlord in the future. I certainly hope this never happens for you.
3. In the absence of evidence to the contrary, the safest course of action is to assume that any device, hardware, and fixture that contribute to humidity is potentially causing, contributing to, and/or exacerbating the problem. This includes two large volume aquaria.
4. While steps to reduce humidity such as adding a dehumidifier are wonderful and needed, you also might want to seriously think about reducing non-essential sources of humidity. Your aquaria, will lovely I'm sure, are not essential. This is in addition to finding any fixable issues like a pipe leak as others have mentioned.
5. The money you spent on the aquaria is already spent. You need to think about whether continuing these tanks, going forward, is worth the potential costs. Take into account the fact that you and your landlord had to have serious discussions about putting in these tanks BEFORE this problem started, your landlord will not look very kindly on this new development even if it is no fault of yours. If there is a potential for dispute, you really need to consider how much you are willing to lose. Do not fall into the Sunk Cost Fallacy and keep thinking how much you've already put into the tanks. That money is already gone.
Just my 2 cents.