i've recently read through a few of these threads and played this game some and i have a few thoughts

i will update this as i go so please check back

i am aware that this game has low revenue, but there are a few things available i believe may help increase this

1. [Partner with aquarists on social media] Your target audience is aquarium enthusiasts, so why not actually target them? Partnering with people in the aquarium, bio-terrarium, vivarium and paludarium hobbies will likely catch you a relatively cheap sponsorship as a few factors may help, mainly their interest in seeing such a lovely game that represents their hobby

2. [Welcoming (more) volunteers] I'm not sure if you already do this but some people may want to help, including coding volunteers will increase workforce and reduce cost of development, granted you are kind to your volunteers

3. [Allowing Modding] I've seen this mentioned before but if modding were implemented, players will have much more content and will likely lay off a little about times between updates but this wouldn't be a good excuse for much longer times between updates.
tying back to the volunteer thing, you would be able to contact mod devs about trying to make their mod into official content, given they consent to it.
modding also entices users with freedom and flexibility.

4. [DLCs are a potential risk] While everyone can get behind a good DLC or two, too many will infuriate your userbase. essentially mimicking micro-transactions in a way, games like this include The Sims 4 and Planet Zoo, i feel no more than 5 DLCs would be a good number. the price of your game can always increase to match content value, but too high of a price will discourage users from buying

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Feature Suggestions

1. [Breeding Crossbreeds] I love the fact you can crossbreed any fish, but i have so many beautiful crossbreeds that i would love to stick together or with other purebred fish. with the way the breed lab seems to work it wouldn't be too far of a stretch to do perhaps
1 a. [In-tank breeding] having an option to allow or disallow breeding in the tank would be so neat, as seen in other aquarium sims
1 b. [Lineages] In addition to the breeding suggestions, being able to see a fish's family tree leading to them would be super neat.
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2. [Campaign / Tycoon mode] I can see campaign mode is apparently planned but heres a few suggestions on how to go about it.

Campaign/Tycoon would likely sit around being a hobbyist and what do those hobbyists do sometimes? they take commissions! this mode would likely be the user starting out with one fish tank. Starting with your basic fish tank. this would represent your home tank, you get to pick from 4 starter fish; Siamese Fighting Fish, Fancy Guppies, Tetra, or Goldfish. you get a male and female of each species and your goal is to breed them, sell them, make money, and upgrade your tanks and perhaps skills

FEATURES;

- [dynamic backdrop] backdrops would potentially hold a lot more depth for this mode. since you are a hobbyist this backdrop will either be an office, bedroom, or hobby room. this would be built like any other room in a game, but wouldn't be able to be interacted with as it is just a backdrop. however, this means you can add things to shelves. said surfaces could hold size-accurate fish tank assets for ambiance, potentially progressive so they show up as you get them

-[Rewarding play] to tie into above, trophies and achievements would help reward players in a visual way as a trophy cabinet or shelf where trophies gained will sit there in a players view

- [Social Media] its all the world is today, embrace that in your game. You could be on or own a website where you sell your fish and tanks. customers can submit commissions you can reject or accept, you can list your fish and tanks with custom prices

- [The End] There would need to be a technical 'end goal' of the game. this could be getting a certain award, getting to own a fish store, or even just becoming famous online for your fish. but the end goal shouldnt be the end, people love replayability

- [Different Specialties] This one is a bit more advanced, but having there be different ways to specialize in different fish would be pretty neat

- Generalist - starting path, no buffs

- Saltwater Specialist - specializes in saltwater fish, picking this will allow the user to start with saltwater fish instead of the freshwater starters, the player has an easier time monitoring saltwater aspects of the game like salinity, coral and sea critter, easier to keep saltwater fish happy

- Freshwater Specialist - Player will have an easier time with freshwater aspects, allows player to sell the cheaper than saltwater fish for more as a higher quality fish

- Carnivore Specialist - Player has an easier time caring for carnivores, they breed feeder fish much more quickly and can fit more feeders into a feeder tank, carnivores are easier to breed and less aggressive (Herbivores are harder to care for

- Herbivore Specialist - player can monitor plant and algae growth more easily, plants are easier to care for and herbivores require less food (carnivores are harder to care for)