Inactivity in the tank and unstable temperature

Question
hello
  i work in a petstore myself for 1.5 years. i have a 20 gallon tank that has been set up for about a year now. i occasionally clean it (once in two weeks) i feed my fish twice a day: mornings: regular flakes; evenings: color flakes and occasionally treats like ocean plankton or bloodworms. during the night i drop a veggie wafer for my pleco "Slimy"
  i have 14 fish in the tank. 8 blackstriped tetras. 2 red eye tetras. 1 marble sailfin pleco, 1 tiger barb and 2 swordtails (male and female).
  my filtration system is a "penguin filter" that runs with a pad with cotton and activated carbon and bio wheel, i also have an airstone that works properly.
 the water is great, no problems with algae or anything, no clouding.
 i have two problems with it, hopefully u'll be able to answer.
 PROBLEM #1 my only fish that are active all the time are the pleco, barb and swordtails. all other fish are inactive and just hang out in once place and active only during the feeding.
what could be the problem?
 PROBLEM #2 the other problem is that the temperature is always below normal.NOTE: the fish were inactive even before there were abnormalities with the stability of temperature. right now it's only 68-69F and has to be 75-79F. do u think it's the problem with the heater or the environment around it. the aquarium is in a dark spot in the house away from heat sources and windows.
                      Thank you in advance
                       Hope for ur soon reply

Answer
Hi Dima;

It sounds like you either need to turn the heater up, the heater is too small for the tank or the heater is defective. All your fish are tropicals and need a temperature of 75 to 80 all the time. They will be lethargic if it is lower.

The general rule of thumb for heater power is 5 watts per gallon. Your 20 gallon needs a 100 watt. If the one you have is lower than that get another that will add to the wattage to make 100, or just get a single 100 watt heater. If your home tends to be on the cool side (below 65) where the tank is then get a 150 watt.

To test your current heater, gently and lightly feel the water close to it at the bottom half (where the heat coils are) while the heater indicator light is on. Do not remove the heater from the water to do this and don't grab it or you could burn yourself. Just feel around it for heat while it's under water. The heater should always be heating up when the indicator light is on.

Followups welcome

At Your Service;
Chris Robbins

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