# FAQ Details

## Sand and gravel in combination? / How much sand is required?

Hello,

*I will purchase a 250 liter aquarium and would like to put undyed reddish brown gravel (3 - 4 mm) and, in a small spot, white sand into this aquarium. Is this OK for Coolie loaches (they need sand after all)?*

a small sand zone is not of much use, and the sand would literally sink into the gravel within shortest time. Also, the Coolie loaches require a larger sand area. I recommend to set up the aquarium entirely with sand or fine (!) gravel and possibly distribute some of the coarse red gravel on top (please refer to our booklet "The Wellness Program", exactly this solution is presented in the "Biotope Aquarium 'Asia'", which also includes Coolie loaches).

*And are** 25 kg **(not including sera floredepot) of sand and gravel in total sufficient, or should I purchase more or less gravel/sand?*

This is very easy to calculate, at least for rectangular tanks: Length multiplied by width (each of them in cm) divided by 1000 equals the liters per centimeter layer thickness. The bulk density of quartz sand/gravel is about 1.6 - 1.7 kg/l, which allows to easily calculate the weight as well. A 250 liter aquarium with a bottom area of 100 cm * 50 cm thus comes up to 5000 cm² bottom area, and 5 liters or 8 - 8.5 kg sand per 1 cm layer thickness. 25 kg of sand are therefore sufficient for a layer about 3 cm thick. I therefore recommend doubling the amount to 50 kg in this case.

The calculation is of course an approximate value as it, for instance, ignores the glass thickness when calculating the net bottom area, but a pound of sand more or less is not that important.

Best regards

sera GmbH

Dr. Bodo Schnell