The proper way is to anneal is to put the tank in the oven at a specific temperature, unfortunately a lot of acrylic tank manufactures don't have this equipment as it expensive, let alone having an oven big enough to fit a tank like Peter's .
Here is some info
Proper annealing is one of the most effective single measures that can be taken to insure good service from parts made of
Plexiglas® sheet.
Annealing consists of prolonged heating of the acrylic part at temperatures lower than those used for forming, followed by
slow cooling. Internal stresses set up during fabrication of the article are reduced or eliminated by this treatment All edges
should be cut or machined with as little internal stress as possible. If excessive internal stress is present in the parts to be
cemented, crazing will occur during the cementing process. It may be necessary to anneal the parts to reduce internal
stresses that set up in the parts during fabrication or thermoforming operations. Annealing results in greater dimensional
stability and greater resistance to crazing. Heat treating also improves the strength of certain cemented joints.
To obtain the benefits of annealing, Plexiglas® sheet parts must be annealed after all fabrication steps. including polishing,
are completed. In addition to annealing after final finishing, machined parts should be annealed before cementing to reduce
stress due to machining in the cement joint area.
Here is a PDF
http://www.plexiglas.com/export/sites/plexiglas/.content/medias/downloads/plexiglas-expert-pdf/Fabrication-Guidlines-Plexiglas-Annealing.pdf