75 litres is not 45 gallons... I think there are approximately 4 litres in every gallon so my original estimates must have been correct (please look at the links thapsus posted). 39 inches x 16 inches x 18 inches cannot possibly be 45 gallons. That is approximately 50 cm x 30 cm x 35 cm... sounds like a typical 20 gallon to me.
What happened to all your other fish? You cannot keep them all in the smaller tank. You should (gradualy) devide them between the two and don't add any more unless you are certain (by testing for nitrates) that your tank is not over-stocked.
Also make sure the tank is cycled (0 ammonia/nitrites, high nitrates), before adding ANY more fish and do a large partial water change to remove most of the nitrates before you do.
You should be able to keep nitrates at a level just above that of your tap water with one weekly partial water change of around 20% - 30%. Adding plants will increase the time you can wait between water changes.
Consider the 1 inch of fish per gallon guideline when calculating the number of fish you want to keep... ALWAYS use adult sizes and take note of the species you are buying and research its requirements BEFORE purchasing it. In a 20 gallon tank you can keep up to 20 inches of fish (adult size). Obviously, this is only a guideline (you couldn't keep a single 20 inch fish in a 20 gallon tank! though you could keep 20 neons for example - without any other fish of course... or 6 neons, a dwarf gourami and 4 cories).
Personaly, I don't like this guideline but it may be useful for you.