The main reason for filtration in any tank is to keep the water clean of waste and larger food particles. the filtration traps the larger particles that float free in the tank. With good filtration most fish and plant waste is picked up and removed. The other part of filtration is to insure a Bio filtration system that maintains the bacteria home that allows toxic ammonia to be broken down into Nitrite and nitrates . The nitrite is broken down into usable Nitrate for plants to feed on.

So I can only say that a non filtered tank has no way to cycle and maintain water parameters without having to change larger quantities of water per week or every other day. It's up to the individual on how much time they want to spend on tank maintenance.

Me for one I'm lazy and want to have more time spent on watching my fish instead of doing water changes. If there is no fish in the tank and it's just a Nano Planted tank then no filtration would be needed.

 

Its quite possible to have a small filterless tank. Its not at all suggested if you need it heated though.
I have a 2.5g tank that is all plants and a single betta. The water parameters are flawless and I change it once a week, about 50%, and vacuum the tank every other time. Dirt is okay. Dirt breaks down from both detritus worms and bacteria. Plants eat the nitrates, and the cycle is complete with new water every week. It takes Way more time and care to set up a filterless tank, and expect more then three months of daily checking of water and water changes every other day. If you have a filter that works with the tank, keep it that way. You won't need to worry as much.
For the heater part: Hot water rises while cold water sink, so if you don't have circulation you can have a huge variation in temperature in just 10g. So no heater if their is no filter since it can cause temperature shock.

 

Will there be a large variation in temperature in a small tank? Would an air pump counteract that? The issue with the filter in my 3 gal is that it’s REALLY big and the inbuilt light is killing my plants off. And I’m unsure about the ehiem minis flow? I feel like it will be too strong..

 

I have 2 filter less tanks.. 1 with heating and 1 without. Dont have problems with water parameters or temperatures.. my guppy's and shrimp are breeding and plants grow. Only thing to look out for is a film on the top of the water. (Shrimp, soil and enough plants are the key. They are your filter..)

 

I'm afraid of shrimps... And they're not essential, anyway. But in general, that's true.

 

Sponge filters are good but if you want something else, Finnex makes a great HOB for smaller tanks. I have one on my 2.5 with no issues. TruAqua also makes on. Both are fully adjustable.

@Rainbo has, I believe, a filterless two gallon tank. So it can be done. Perhaps she'll pop in and let you know how hers is going. It may be snails and shrimp?

 

Yep, I managed to accidentally cycle a 2.5 gal unheated, unfiltered snail tank. It kept the cycle for months, then I decided to put in a heater and filter and added a betta. I never lost the cycle.

If anyone wants to try it start with some anacharis and around 5 ramshorn snails, I used gravel as a substrate but sand should work. Feed the snails every 3 or 4 days. Put the tank near a window so that the plants get some light and do 50 to 75% water changes every week or two. After 2 months decide to test the water just to see how bad the tank is doing and discover that the thing cycled all by it's little lonesome self. I suspect the key to the whole thing was the fast growing anacharis, it acted as a filter and helped rid the tank of some of the ammonia and nitrates.

You can have a heater without a filter if you use something like an airstone to keep the water moving. I don't use filters when I set up a 1 to 2.5 gal hospital tank, but I do use an airstone and heater. I've tried it without the airstone and ended up with the water being warm near the heater but cold at father side of the tank, it can be a 3 to 5 degree difference in temp.