The 300 gallon would be awesome. However the issue with the mix of fish that you have isnt that they aren't just from different regions, they require very different water conditions to truely thrive at their best rather than just survive. As i said im not tellin ya you have to do what i say, in fact i would much rather see someone do the research and see the reason for my advice rather than just taking my word for it.
My advice would be a couple different tanks when you get the money so that you would then be able to set up different tanks for the different environments your fish require.
For example one tank could be for your african cichlids, another could be for your JD and others, and maybe anther set up as brackish for the puffer.
This is excellent advice and it's the same I'd offer. Rather than a 300 gallon tank I'd suggest double stacked 135-150s and do one African and one CA/SA and do like a 40 breeder brackish for the GSP. The double stacked tanks would save some space while giving 2 tanks to create 2 different environments. Of course these are just some ideas for you to consider. Welcome to MFK, there are plenty of helpful folks here. I've been in the hobby for only 2 years and this site has helped a ton. I suggest browsing various threads and reading, it helps a bunch.
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