A 300-gallon aquarium typically costs between $1,500 and $5,000 new, depending on material, brand, and included equipment. Used tanks in good condition generally run $500 to $2,000 depending on age, whether the stand is included, and how urgently the seller needs to move it. If you're actively looking for a 300-gallon tank for sale, Craigslist, Facebook Marketplace, and local reef club forums are where the best deals appear, while Marineland, Aqueon, and custom acrylic fabricators handle the new market.
This article covers what you're actually getting with a tank this size, what to look for in both new and used options, the practical realities of setup and support equipment, and the total cost to expect before livestock ever enters the water.
New vs. Used: Which Makes More Sense at 300 Gallons?
At this scale, used tanks make financial sense if you know what to inspect. New tanks at 300 gallons from major brands like Marineland (their Deep Dimension series topped out around 300 gallons) or Aqueon are no longer commonly stocked at retail. Most buyers in this range are looking at custom acrylic fabricators or purchasing used setups from hobbyists who are downsizing.
Buying New
Custom acrylic tanks from companies like Miracles Aquariums, Acrylic Tank Manufacturing (the ATM brand), or SeaClear can be built to any footprint you specify. A standard 300-gallon configuration runs around 96" x 24" x 24" or 84" x 30" x 24". Acrylic tanks at this size cost $1,800 to $4,000 for the tank alone, plus stand and sump.
Glass tanks at 300 gallons are less common because the glass thickness required (3/4 inch or more) makes them extremely heavy, sometimes 800 to 1,000 lbs empty. Some hobbyists still prefer glass for scratch resistance, but at 300 gallons the weight becomes a real structural concern.
Buying Used
Craigslist and Facebook Marketplace regularly have 300-gallon setups for $800 to $2,500, often including stand, canopy, sump, and sometimes lighting. The key is to inspect carefully before committing. Acrylic scratches; check for crazing (fine surface cracks that look like spider webs) around the corners and seams. On glass tanks, inspect all silicone seams under good lighting. Any browning, peeling, or separation is a red flag.
Before you agree to buy, fill the tank with water where it sits or at least ask the seller to fill it for 24 hours. A tank that's been dry for two or three years may have degraded silicone. Refilling and confirming no leaks before you dismantle and transport is worth the water bill.
Footprint and Structural Requirements
A 300-gallon tank filled with saltwater, rock, and substrate weighs approximately 3,000 to 3,500 lbs. That's not a number most residential floors can handle without assessment.
Most homes with a concrete slab on grade are fine. Wood-frame floors, especially upper stories, need evaluation. General guidance from structural engineers is that residential floors are rated for 40 to 50 lbs per square foot live load. A 300-gallon tank on a 96" x 24" footprint puts roughly 175 to 200 lbs per square foot on the stand's contact points. You'll almost certainly need to sister the joists beneath the tank or install a structural column.
Get a structural engineer to look at your floor before the tank arrives. It's $200 to $500 and can save you from a catastrophic floor failure.
Essential Equipment for a 300-Gallon System
The tank itself is the cheapest part. Budget for the full system:
Filtration
A 300-gallon reef or FOWLR setup needs a large sump (at least 50 to 75 gallons), a protein skimmer rated for the volume, and significant biological filtration. Reef Octopus, Bubble Magus, and Skimz all make skimmers in this range. The Reef Octopus 200SSS or similar models rated to 400 to 500 gallons give you headroom for a heavily stocked system.
Check our Best Protein Skimmer for 300 Gallon Tank guide for specific model recommendations by budget.
Return Pump
You want a return pump moving at least 3 to 5x total system volume per hour through the sump. For a 300-gallon display plus 75-gallon sump, aim for 1,500 to 2,000 GPH. Reef Octopus Varios 8, Jebao DCS-12000, and Ecotech Vectra L2 are common choices in this flow range.
Circulation
Beyond the return pump, powerheads or wavemakers create water movement inside the tank. Ecotech MP40s or Tunze 6105s are popular choices for large systems. Budget two to four powerheads for a 300-gallon display.
Lighting
Lighting a 96" x 24" footprint takes either three to four T5 fixtures running eight or more bulbs each, multiple metal halide pendants, or four to six LED units like Ecotech Radion XR30 G6 or AI Hydra 64. Budget $1,000 to $3,000 for adequate lighting on a reef.
Heaters
For a 300-gallon system, you'll need at least two heaters to maintain stable temperature. Titanium heaters like the Finnex TH-800S (800W) or the EHEIM Jager 300W run in the sump to prevent livestock from touching them. Use two heaters and set the backup 1 degree lower so it only runs if the primary fails.
For a full overview of what a large system needs, the Best Aquarium Equipment guide covers everything from skimmers to controllers.
What to Budget for a Complete 300-Gallon System
| Category | Budget Range |
|---|---|
| Tank (used) | $500 to $2,000 |
| Tank (new custom acrylic) | $2,000 to $5,000 |
| Stand and canopy | $500 to $1,500 |
| Sump (50 to 75 gallon) | $200 to $600 |
| Protein skimmer | $300 to $800 |
| Return pump | $150 to $400 |
| Wavemakers (2 to 4 units) | $200 to $800 |
| Lighting (reef) | $800 to $3,000 |
| Heaters | $100 to $200 |
| Salt and initial water | $100 to $250 |
| Rock (150 to 200 lbs) | $300 to $800 |
| Sand (100 to 150 lbs) | $100 to $300 |
Total for a used tank with new equipment: $2,750 to $7,650 Total for a new custom tank with full equipment: $5,000 to $13,000+
These are ballpark figures. Many hobbyists keep costs lower by buying equipment used through reef forums or upgrading in stages.
Where to Find a 300-Gallon Tank for Sale
Facebook Marketplace is currently the best source for used tanks. Search "300 gallon aquarium," "300 gallon reef," or "large fish tank." Filter by distance and check listings weekly because large setups sell fast when priced reasonably.
Craigslist is still active in most metros. Search "aquarium" and sort by newest.
REEF2REEF Sell/Trade forum (reef2reef.com) has dedicated buy/sell classifieds for reef equipment. Larger tanks appear regularly, often with photos and parameter history.
Local reef clubs often have member-to-member sales before things get posted publicly. Joining your regional reef club is worth it if you're serious about finding quality used equipment.
Marine Depot, BRS, and custom acrylic fabricators handle the new market. Get quotes from at least two fabricators and compare lead times.
FAQ
How long does it take to cycle a 300-gallon tank? A fish-in or fish-less cycle on a 300-gallon takes 4 to 8 weeks. Using live rock from an established system, bottled bacteria products (Fritz Turbo Start 900, for example), and Dr. Tim's Ammonium Chloride can get a new tank cycling in 2 to 3 weeks.
Is a 300-gallon tank harder to maintain than a smaller one? In most ways, larger volumes are more stable and more forgiving. Parameter swings are slower and smaller. The main challenges are physical: more water to change (typically 30 to 60 gallons per week), heavier rock to arrange, and larger equipment to service.
Can I put a 300-gallon tank on a second floor? Possibly, but you need a structural engineer to assess it first. Never assume residential floors can support this weight without verification.
What fish can go in a 300-gallon tank? At this size, you have options that smaller tanks can't support: large tangs like the Naso or Sailfin, adult triggers, large angelfish, and even some of the more active sharks (like the Banded Cat Shark in a species-only setup). A reef version can house a diverse community of medium fish at reasonable stocking levels without territory issues.
Conclusion
A 300-gallon tank is a serious long-term commitment in space, budget, and time. Used tanks make economic sense if you inspect seams and verify no leaks before transport. New custom acrylic is the right call if you want a specific footprint or can't find a used tank in good condition locally. Don't underestimate the floor load requirements and get that assessed before anything arrives. Build your equipment list from the sump up, and budget the total system cost rather than just the tank price.