App Overview

RYUKIN icon

RYUKIN

Masataka Hakozaki · Entertainment

~ Gracefully Fluttering Goldfish ~

$3.49 · In-app purchases

PRICE
$3.49
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
4.9
61 ratings · Top 5%

Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.

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Build Kit

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A product spec, screen plans, and build steps for your coding agent.

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What's included18 files

A product spec, phased build plan, differentiation notes, and screen references.

  • START_HERE
  • AGENTS
  • EVIDENCE
  • SPEC
  • BUILD_PLAN
  • DIFFERENTIATION
  • skills/
  • screens/
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Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.

Feature spec

Goal: Send a clear invoice after finishing a job.

  • Add a client, line items, a due date, and payment instructions.
  • Preview the total before exporting a PDF.
  • Keep an editable draft if export fails.

Screen plan

Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.

States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.

Differentiate and validate

Hypothesis: Contractors need faster repeat invoices more than more templates.

First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.

The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.

Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.

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Quick prompt

> SELECT BUILD PLATFORM:

Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.

// RORK SYSTEM PROMPT
Rork plan preview for RYUKIN: map the core Entertainment workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
            

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Opportunities

AI-GENERATED OPPORTUNITY HYPOTHESES

These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.

Hypothesis 1

Provide a broader selection of genres and categories.

Assumption to check: Limited content variety

Validate with people who use this workflow before building.

Hypothesis 2

Implement offline viewing for downloaded content.

Assumption to check: No offline mode for downloads

Validate with people who use this workflow before building.

Hypothesis 3

Create an algorithm for personalized content suggestions.

Assumption to check: Lack of personalized recommendations

Validate with people who use this workflow before building.

Hypothesis 4

Simplify the in-app purchase process and offer clearer value.

Assumption to check: In-app purchase complexity

Validate with people who use this workflow before building.

App evidence

Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.

App Store description

~ Gracefully Fluttering Goldfish ~ Goldfish glide gracefully through crystal-clear water, their fins fluttering gently as they swim. Their charming gestures and soft, delicate fin movements have been carefully recreated. Enjoy the beauty of goldfish anytime, anywhere — without the need for water changes or feeding. App Highlights - Fish respond to your touch and follow your finger - Customize tank shape, background color, and floating objects - Soothing water sounds and clear, refreshing visuals - "Cat & Baby Mode" for safe play with kittens and toddlers - Sleep timer for relaxing bedtime use - Play your favorite music while watching the aquarium - Clock and date display - White and black goldfish included; colorful and themed fish available via in-app purchase Settings & Customization - Camera options (Free position / Follow fish / Auto-rotation) - Screen orientation - Tilt sensitivity - Sleep timer - UI size - Toggle water surface, bubbles, and other effects - Adjust brightness, saturation, contrast, and color tone - Quality settings (resolution and various effects) - Volume settings (water sounds, bubble sounds) - Clock & date options (font, position, size) Other Features - Fish, objects, and the water surface respond to touch and device tilt - Fish may whimsically swim toward your finger - Tap too much and the fish may faint! - Feed the fish - Adjust the size of fish and objects - Display up to six fish at once ‥∴ Bringing the Allure of Goldfish Closer ∴‥ This app was born from a childhood dream of recreating goldfish through programming. Fifteen years after the release of the original app *Wa Kingyo*, countless refinements have been made to capture the graceful and charming nature of these fish. From the gentle movement of their fins to the way they chase and munch on food, experience the beauty and warmth that goldfish bring. News and Support X: @m_hakozaki TikTok: @m_hakozaki Instagram: @masataka.hakozaki

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Screenshots

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Build scope

Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.

Define the first version

Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.

Set boundaries

Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.

Check the hard parts

Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.

SUGGESTED BUILD STACK
React Native
Expo
Supabase
React Navigation
Zustand
expo-video
BUILD KIT DIFFICULTY
Advanced

From the Build Kit assessment.

A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.