How often water change 70ltr tank?

[fluttermoth]

Adding filtration won't increase the amount of fish you can keep.

The amount of fish you can keep in a tank depends on; how much swimming space the fish need, how big they grow, how many you need to keep in a group for the fish to be happy, and how fast pollutants like nitrate (which we can measure for) and fish hormones and pheromones (which we can't) build up.

Adding a second filter might mean the ammonia the fish produced is converted to nitrate more quickly, but the amount of total waste will still be exactly the same, and will still have exactly the same effect on your potential stocking levels.

 

[NickAu]

Great advice everyone.

Never be afraid of doing a water change, there is no such thing as too much clean water.

My 2 foot tank is what some people would consider way overstocked, 1 Male Betta 15 Kuhli Loaches about 70 Red cherry shrimp, a baby BN, 2 mystery snails and an unknown number of Malaysian trumpet snails ( well over 100 ) my ammonia and nitrites are 0 and my nitrates are about 10 after 7 days and all I run is a small hang on back filter.

My 6 footer has 5 female bettas, 40 Kuhli loaches, a full grown male BN, 5 Mystery snails a bunch of Cherry shrimp and of course an unknown number of Malaysian trumpet snails, It used to run a canister filter till it broke down now all it runs is 2 large sponge filters without any problems.

 

[AbbeysDad]

the aquarium shop told me because I am running two pumps I am not over stocked. The tank does not look over crowded although I am not an expert

Filters are great. They remove and trap detritus and uneaten food, provide an excellent platform for beneficial bacteria, and circulate water to distribute nutrients and minimize thermal gradients. But they are also very misunderstood. You hear all the time that you need 4 to 10 times the tank size in gallons per hour flow rate and that there's no such thing as too much filtration...and these are both repeated myths.

As good as the filter is in getting the water to 'look' clean, the organics inside the filter decompose to pollute the water. It is only when detritus is removed from the system that there's any hope in aiding in purification. But filters are often not serviced enough for this to happen. So they run and run and become pseudo nitrate factories.
I've heard some complain that fine filter media too quickly slows the filter flow, 'forcing' them to service the filter too often, so they choose not to use it, What?

So does using a more powerful filter or adding another filter really 'clean' the water any better? It may seem that way, but actually not really.
There's one possible exception I'm willing to concede. Immediately following the weekly water change I have been running a Marineland Magnum Internal Polishing Filter with the micron cartridge charged with diatomaceous earth. I let this run for several hours to 'polish' the water to crystal clarity. This process removes disturbed algae and dissolved organics from the water column.

But the real secret to good filter/tank housekeeping is to service the filter frequently to get the decomposing waste out of the system.

 

[Byron]

the aquarium shop told me because I am running two pumps I am not over stocked. The tank does not look over crowded although I am not an expert

This takes some explanation, and I did some of this in post #10, but will add a bit more now.

Filtration is only part of the balance in an aquarium. If the tank is properly balanced you don't even need a filter. I am not suggesting you do that, but the point is that the filter is not a crutch to lean on, that will be a disaster if it were suddenly not working. It is an aid, but not what holds everything together.

As for stocking, as I pointed out, you do not have sufficient numbers for most of the species, so that affects them and they in turn affect the biological system. Getting more to solve this one problem would be way too many in this tank, so the only solution is to remove some species and stay with others.

At this point we come to the adult size. Fish grow continually, and in two ways. Outward or external size, and internal organ development. The two should function in sync, but the environment (tank size, water conditions, numbers, stress) can affect either or both. And any detrimental factors will be serious for fish. Fish are very much different than any land animals in this aspect.

So image the present fish at their expected mature size, and with the necessary numbers of each species, and you have a tank that would practically be wall to wall fish. You cannot rely on most fish store staff to work this out, only your own research.

Edit. There were a couple of posts after the one I was responding to here, that I didn't see (my fault) so I repeated some but that's OK. It all needs saying and accepting and understanding.