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There are after all so many decisions to be made as to the type of fish you would like to keep, how much work is involved in maintaining and even the types of food that they should have.
Consider the purchase of your fish as an investment and remember that time spent planning and researching in the initial stages can make a great deal of difference to the success of your aquarium going forward. When choosing your freshwater aquarium fish it is worth you considering the following points: What size tank do you have? If you have a small tank then you will need smaller fish and fewer of them.
Just remember that some fish quickly outgrow their tanks so double check their average size prior to purchase. If you are just starting out, make sure that you choose the largest aquarium tank that you can have within your chosen room and then make your decision on the fish that you hope to purchase. There will no doubt be many freshwater aquarium fish that will appeal to you but you do need to ensure that any larger fish will not prey on any smaller ones or torment them. A little research goes a long way and can prevent the loss of any of the inhabitants through bullying.
Check if the fish will be territorial and aggressive or fairly passive.
Again, check prior to purchase as some species simply do not gel well together. Do you wish to have a species orientated aquarium or a community of fish? Try to visualize how you want your aquarium to look.
A community of fish that blend well together can be lovely to watch because of the diversity of species but a small shoal of one species can also have its advantages. Think of costs too. Some freshwater fish are readily available and inexpensive to buy whilst others will be much more costly. In addition to buying your fish, make sure you have brought the relevant food stock as some species require different food types and ensure that your aquarium is completely prepared for the new arrivals.
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From: (tropical fish aquarium) http://aquariumnation.com/freshwater-fish/choosing-freshwater-aquarium-fish/