Fungus and Mould

The most functional solution  to avoid humidity excess and moulds, is to create a moderate air flow from the bottom to the top, this prevents  humidity excesses and stagnant air, two main causes of plants disease. CPUs fans are unusable for Terrarium because a contraindication as too much intense air flow that could dry up some parts of plants, evaporating water might cool plants that may suffer, CPUs fans are not designed for working at high humidity levels and often sprinkle oil and lubricating substances. But if you are intentioned to use a fan, buy an hi-quality one. 

A way  to generate an almost natural air flow is exploiting the "chimney effect". 

How it works. Hot air and tends to flow toward the rear aperture causing a pressure reduction in the lower layers establishing a continuous air flow.

Building  a Terrarium using the above system  it's easy. The  draw below shows as build a tank suitable as terrarium:

In figure there are not shown:

- The main cover. It can be an aluminium sheet with proper mounting.
- The cover used for electrical fixtures as lamps, ballasts, fuses, etc. It is placed just above the opening between
   "A" and "G".
- The frontal sliding door.

Please note that are 6 holes. Three in side "B" and three in side "E". These holes and  the opening between "A" and "G" allow air to circulate. How does it work? Fluorescent or discharge lamps placed below the hat  heats air, hot air tends to flow out the opening between "A" and "G" and new cold air flows from the below holes.
This system allows  to close the terrarium with an additional sliding glass sheet (not drawn). In this case temperature won't
rise over  10% depending from hole and upper opening size.

During the night, when lights are switched off,  cool and wet air will lay on the peat simulating the nights in the wild.

As the Terrarium starts to work it may produce a bit of fungus or mold that will disappear after a few treatment with very light fungicide. Good rule for outdoors Terrarium is to cultivate live Sphagnum due to its antibiotic and antiseptic properties.

If you want to start with a commercial fish tank you may be in trouble due to high humidity level. Water could condensate on the internal windows causing harmful effect for most of plants.

A solution is asking to a glassworker to lower  10-12 cm from the top one of the main windows as shown below:

This is the best solution for building a Tropical Terrarium . 

Watering System

On the bottom of the tank there is a sort of  "mezzanine"  drilled  of small holes, it lays  on plastic wedges, a bed of inert material as perlite sparates cultivating medium by mezzanine hence by water.

 See picture below:

Yellow circles are the plastic wedges which serves as support.
The upper red line is the mezzanine, between the red lines there is water.
Between upper red line and lower blue line is perlite.
Between two blue lines the growing medium.

 A pump provides to create a water flow. You'll have to shape mezzanine creating an opening protected by walls able to hold pumps without letting peat to fall into water,  hidden  pipes brings water from the well  to the opposite corner through a bigger hole drilled in the mezzanine.

Waterfalls are made using acquarium logs. This particular wood it's very strong and if properly treated never cause problems as rottenness, fungus, mould. To prepare woods, first put  in a basin with tap water for several days changing water every 12h, you'll se water became tea like. When logs won't dirty water, put them into boiling water for a few minutes, this should destroy mould spores. Therefore choose the position into the Terrarium. To increase water oxygenation it is possible to recreate artificial waterfalls or water dripping through woods.

 Finally cover all pipes with growing medium.
Under the drippings, where water drains, make a hole into peat  upper the mezzanine and cover the internal edge of the hole with small stones ( use quartz or similar ) .
Pumps should  always work. Check water acidity monthly, average pH values are  4 to 5.

In any case don't forget to sprinkle water from overhead at least every 20-30 days.