Honestly I don't see any real way that these two things mesh at all. There's no particular need for decentralization of "money" storage and exchange in the aquarium hobby, nor for pseudo anonymity.
Are you referring to just a 'blockchain application' or a cryptocurrency?
If you're referring to just a blockchain application, that runs on an existing blockchain but isn't it's own new cryptocurrency, then I'm just not imaginative enough to come up with any purpose for this or any reason for it to exist.
If you're referring to a cryptocurrency, the only thing I could think of that might turn out to be kinda cool is using it for private sales of corals and equipment amongst hobbyists, like 'Hey I'll sell you this coral for 5 reefcoins' type thing. If it took off in popularity in the hobby, you could have a nice ecosystem where people sell their used and no longer needed equipment, or corals, maybe even services for reefcoins (I'm just going to refer to this hypothetical future cryptocurrency as reefcoin).
The problems you'd face are:
Valuation - how do people value the coin and the things they're selling/wanting to buy? What about fluctuation in value? What if some outside entity starts speculating with the coin?
Getting enough people to adopt it, because why not just keep using money as we have been doing? Well I can see one point to it - I've already spent that $$ on this skimmer I'm no longer using so it's a sunk cost. If I sell it for $$, I'll get some back but I might then end up spending it on things not aquarium related. If I sell it for reefcoins, then I've just got reefcoins in my wallet waiting for me to spend on some corals or other piece of equipment someone else is selling. It takes the 'gotta keep myself from spending this money on other things' bit out of the equation. I could see where some people might find this attractive. Of course there needs to be a large ecosystem of hobbyists accepting reefcoin for their stuff for this to be in any way useful or desirable.
You'd also have to have mining to some extent, for initial coin to get out into people's hands, because just selling coin to people would probably not take off - here's this new coin that you'll be able to use to buy from other hobbyists in the future, trust me! and buy coin from me! .....yeah no.
You also would have to limit mining in some meaningful way, otherwise you'd have people just set up mining farms (not on the scale of bitcoin or ethereum, but still) to get as much coin as they could so they could buy things from other hobbyists for essentially free (except the cost of the GPUs, which they may already have, and electricity). If enough people did this, it would hinder adoption because inflation. The easier it is to get coin, the more coin people will want for their stuff, the more you need to get for your stuff in order to get a fair value in exchange.
The more I think about this the more I think these things just don't really fit together. I'm not saying it couldn't work, but there's a lot of challenges to overcome.
