Can I still put a heater in my fish tank?

Question:

My new 5 gal tank is almost done cycling according to my results. I've had it for three and a half weeks. I've kept two Guppies in there ever since I got the tank, and three days ago put a Platy in there with them. The tank didn't come with a heater, so I haven't been using one. But it did come with a filter and a light. I have gravel, and a plant in there along with the fish. I was going to buy a heater this week for the tank, but I was wondering if the sudden heated water would stun the fish? I'm not sure if they're used to the water just being the way it is because all three of them are acting normal. Also, I found a rock outside I would like to put in the tank. I washed it off, and scrubbed the dirt off with a toothbrush. Would it be okay for me to put that in there or should only gravel be in the tank?

Answers:

For the rocks: take some vinegar and drop some on the rock. If it bubbles up or frizzes, then its not good for the tank. personally i would had the heater since day one. you can still put one in as it will heat the water slowly. Just don't turn it up real high. Do you have a thermometer? If not, get that too. don't get one of those ones you plaster on the tank side. Get a old fashion metal one that hangs. This way you can put it in and take it out whenever you want. for tropicals the water should be a safe 76 degrees F. and no warmer then 80, depending on he fish. if you keep up with the types you have, then 76 is great.

The fish you have bought are tropical fish. They need a heater or they will die. When you buy a heater make sure you buy a thermometer to make sure the water does not get too hot or cold. They are very cheap and the pet store. The heater will heat gradually and not harm your fish.

You can add a heater, just make sure to slowly up the temp by a couple degrees daily. As for the rock I hear you can test it with vinegar and if it foams up it'll effect the PH. You can probably do it anyway, but it's generally easier to go with what you can get at a pet store.

you should have a heater. remember there tropical fish. the water temp should be at around 77. as the rock goes if you scrubbed it with soap no, if theres no chemical like soap then yeah, go ahead.

Good luck!