Would Reef salt be better than Aquarium salt during the water changes? There is no rift lake salt around us so we've been doing the heaps of aquarium salt.
Yes reef salt is better than aquarium salt because it has calcium and other minerals in it. Whereas aquarium salt is just sodium chloride. And as Byron said, you need to increase the general hardness, so the reef salt used at 1/3 to 1/2 strength will increase the general hardness and the sodium chloride (salt) level in the water. This level of salt will kill most live aquarium plants so if you have live plants, use reef salt at 1/4 to 1/3 strength.
You need to make up marine/ reef salt 24 hours before you use it. You add the salt to a bucket of water and aerate for 24 hours, then you can use it for water changes. If you have asthma or any breathing problems, wear a paper dust mask when handling the reef salt because it is very fine and can cause you to cough if inhaled. It's not poisonous but it can irritate lungs and sinuses for a bit.
If you do add reef salt you can reduce water changes to once a week. However, if you get an ammonia or nitrite reading, then you need to keep doing big water changes and reduce feeding.
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The other thing you can do is add something calcium based (like Byron's suggestion of a calcareous substrate). You can use shells, dead coral skeletons, limestone or sandstone rocks to help raise the pH and hardness. You can buy bags of white shells for use on the bottom of bird cages and these shells can be rinsed and put into a bag that is kept in a power filter. You can do the same thing with coral rubble, available from most petshops in the gravel section.
Calcium based rocks and shells take time to increase the pH and hardness and the more of them that you have in the tank, the faster the water chemistry will change. However, you do not want the pH to change too rapidly because it can be bad for the fish. Having said all that, some shells or limestone would help make the water better for the mollies.
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If you can't find a Rift Lake conditioner at the petshop, see if they can order you some in. Make sure you get a conditioner and not a buffer. The Rift Lake conditioner has all sorts of minerals in it but mainly calcium and magnesium. The Rift Lake buffer is mainly baking soda.
If the local petshop can't order you any in, you can buy it online from numerous places. There are heaps of different brands but they are all pretty much the same. Just find one that has lots of calcium and magnesium in and use that.