12-13-2010, 12:15 AM
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The chemical cons will be a big determining factor over what animals can live in that tank, if any... so I wouldn't be too quick to dismiss those things. The list of things you mentioned before your "yada yada" makes a huge difference in life or death of animals, please don't take that lightly.
Scratching glass is not the only concern with the glass. What if you should need to remove this structure at some point? That would be impossible. How much weight sits in a given area of the tank will also make a difference if its overdone, especially in a smaller tank. Too much weight on one end can cause a lot of pressure on the glass and while it may not happen right away, it can lead to future problems such as cracking and leaking.
Nitrates is a huge deal because in high amounts for any length of time it is toxic to all fish and inverts. Algae would be the least of the worries. There are other chemicals besides nitrates, along with many minerals that can cause a lot of harm in an aquarium.
Texture was not mentioned. Concrete and cement mixes tend to be very rough in texture, which is not ideal for a large number of species of fish. Catfish is a big category, eels, loaches, and even plecos can suffer irritation, tissue damage, sores, etc. from being exposed to a rough surface on a regular basis.
If you can give an idea of what you are trying to create and what animals are planned to be in the tank, it would be much easier to help with your question and also offer alternative, safer solutions.