buying a microscope
Written by Frank Prince-Iles
An essential piece of equipment
Without a microscope it is simply impossible to tell the difference
between a water quality and a parasite problem. The microscope should be
considered the most basic of tools in fish disease diagnosis - indeed,
an accurate, full diagnosis of the disease and its cause just isn't
possible without a microscope!!
A modern monocular
Why is it so important?
Unusual behavior such as heavy breathing, rubbing, flashing, lethargy
is often taken as a sign of parasite disease - yet the same behavour can
be due to water quality problems, internal organ disease or many other
causes. Without a microscopic examination of both the skin and gills it is
simply impossible to tell the difference and any treatment undertaken is
based on nothing more than guesswork! While simply taking a guess as to
the cause and treatment required may work it is just as likely to make
matters worse. So microscopy is a vital and basic step in diagnosing and
treating fish disease.
Buying a microscope
They come in a wide range of types and prices, costing less than a
hundred to several thousand pounds - although fish keepers generally do
not require an expensive model. There is also a market for good
It really depends on what you want to use it for. Most of the common
parasites such as flukes, white-spot and Trichodina can be easily
identified with the cheaper models, but a better quality model is required
for critical examinations of cell structures and some small parasites.
Generally speaking as prices increase you are paying for better
engineering, illumination and optics
Models for fish keepers
There are two basic styles of microscope available. The cheaper monocular
model has a single viewing tube (as above) - which is fine for occasional
use. The binocular models enable you to view with both eyes, giving
a better field of view. If you want to take photos or video - then you
will really need a trinocular model with a dedicated phototube.
Two other considerations which can make a considerable difference are
illumination and the stage.
The better the lighting - the clearer the image. Most of the cheap
models have an understage mirror which reflects light to illuminate the
slide. This can pose problems in dull conditions or lead to contortions
with table lamps to try and improve the illumination of the specimen being
studied. By far the best option is a fixed or plug-in understage light
system, which gives a consistent amount of light. A basic plug-in system
can be purchased for under £50 and, in my opinion, is money well spent.
More expensive microscopes have built-in halogen lamps with brightness
The stage is the part of the microscope where the slides are placed for
viewing. As you will appreciate, only a small part of the slide can be
seen at any time and the slide needs to be moved to see other parts.
Incidentally, the view that can be seen at any time is called the field
of view, which reduces at higher magnifications.
On the cheaper models the slide is held by clips and has to be moved
manually - but this cannot be done smoothly and it is virtually impossible
to return to a given position on the slide.
All but the most basic of models have a mechanical stage fitted with
vernier screws that allows you to move the slide smoothly and examine it
in a methodical pattern. Again, this usually adds less than £50 to the
basic cost and is invaluable when scanning the slide for parasites.
There are different qualities of mechanical stage- and some of the more
robust stages can be quite expensive.
needs to be as thin as possible
Before any specimen can be examined under the microscope, a slide has
to be prepared. No matter how good your microscope is, the final image can
only be as good as the slide you are viewing, so proper preparation is
A normal compound microscope works by passing light up through the
viewed specimen, so it is important that the sample or specimen is a thin
as possible. This means working with relatively small amounts of algae,
mucus, sediment or whatever. This is particularly important when viewing
mucus sample as some of the smaller, transparent parasites might not be
seen if the preparation is too thick.
Slide and cover glass
Ideally a new slide and cover glass should be used for each
preparation, but in practice slides and slips will be re-used many times.
In which case it is important that they are clean and free of smears (
cleaning with alcohol will help remove smears - propan-2-ol from a chemist
or drug store).
- The specimen is placed, together with one drop of pond water onto
the centre of a clean slide. Do not use tap water or distilled water
as these may kill any parasites present. If the specimen is thick, use
a seeker needle to gently spread it as thin as possible.
- A glass or plastic cover slip is then lowered gently on top making
sure that no air bubbles are trapped. The best way to place a cover
slip is to hold two opposite edges between first finger and thumb.
Holding the slip at a 45o angle, place the bottom edge on the slide
just to one side of the specimen and then slowly lower the cover slip
until it is flat. You can use a seeker needle to help lower the slide
by placing it under the cover glass and slowly lowering it into
- Once the cover slip is in position apply a small amount of pressure
with a seeker needle to spread the sample under the slip and squeeze
out any air. Don't apply too much pressure; just enough to spread the
sample. Do not use your finger as a finger print may contaminate the
- Practice makes perfect - so try making slides of all sorts of
things; blanket weed, algae, mulm etc.
sequence of steps to take
There is a clear sequence of steps to take to achieve perfect viewing.
These are detailed in the box below. A word of warning: Take care when
adjusting the focus knobs that you do not advance the objective lens onto
the slide! It is very easy to break the slide and possibly damage the
When setting up the focus it is best to view from the side and lower
the objective so that it is nearly, but not, touching the slide. Now
adjustments can be made while viewing through the eye-piece and slowly
winding the objective UP and AWAY from the slide - this way
you avoid any potential damage to either the slide or microscope.
Getting things in focus
Once the specimen is in focus, fine adjustments in illumination and
iris aperture can be made to improve viewing.
The slide should be scanned systematically, usually by finding the top
corner of the cover glass and then moving the slide slowly across the
stage to the adjacent corner. When the opposite side is reached the slide
is moved up until a new field of view is visible and then moved slowly
across to the other side. This is repeated until the bottom of the slide
Higher magnification are obtained by rotating the nosepiece turret and
selecting another objective and then re-focusing.
Parasites are often transparent
Since many parasites are transparent to light it is often necessary to
use various techniques to highlight them. The two most popular methods are
phase contrast and darkfield. Both of these methods are outside the scope
of these pages, but essentially they manipulate the light so that
transparent objects are more readily visible. These specialist methods
usually mean adding special condensers of objectives to your microscope.
While these methods are useful they are not essential for fish disease
If there is a problem with viewing any specimens with an ordinary
brightfield microscope it is possible to increase the contrast by racking
down the condenser or closing up the iris aperture, although it does
Read further in Part 2