what am i doing wrong with my fresh water fish tank?

i have a 20 gal fish tank and i been running 4 several months i have a water heater it is set to 78 deg and a whisper 30 filter Machine i was told to get a power head i did several min later 4 fish died. also how often should is chang the water
i was told that if you look down in the tank and can see the bottom then the waster is not being stured up enougt and will not have enougt air. do i need the power head

I don't think you need a power head. I also have a 20 gal tank with a whisper 30. I have had absolutely no problems with mine. I do water changes every week, 25% each time. I change the filter once a month and do a gravel vacuum once every three weeks. I do NOT ever do all of these actions at the same time as it will interfere with natural bacteria levels. Get yourself a water testing kit and start checking your water levels-specifically ammonia, nitrite, and nitrate. That will help you decide when to do water changes and get a book about freshwater fish care.

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 1h 17m Buy It Now for only: $7.99

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 1h 17m Buy It Now for only: $7.99

Ocean Fish Machine Embroidery Designs CD $9.95 Time Remaining: 5h 45m

KIDS FISH DOLPHIN SHARK EMBROIDERY MACHINE DESIGNS CD $9.99 Time Remaining: 6h 26m Buy It Now for only: $9.99

Tropical Fish Machine Embroidery Designs set PES HUS JEF VIP DST $9.95 Time Remaining: 6h 29m

Fishing Machine Embroidery Designs $3.49 Time Remaining: 8h 10m Buy It Now for only: $3.49

Cute Dolphins fish sea creature friends kids machine embroidery designs ANML27 $9.99 Time Remaining: 10h 38m

New beautiful fish sea animal cute kids machine embroidery designs FSH9 $9.99 Time Remaining: 10h 48m

Googly Eyed Fish kids machine embroidery designs FSH8 $9.99 Time Remaining: 10h 58m

Gold Fish machine embroidery designs set 4x4 inch hoop $4.90 Time Remaining: 15h 4m

Gold Fish machine embroidery designs set 4x4 inch hoop $4.90 Time Remaining: 15h 4m

Gold Fish machine embroidery designs set 4x4 inch hoop $4.90 Time Remaining: 15h 4m

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 15h 23m Buy It Now for only: $7.99

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 15h 23m Buy It Now for only: $7.99

Tropical Fish Machine Embroidery Designs set $9.95 Time Remaining: 18h 17m

Tropical Fish Machine Embroidery Designs CD PES ocean clown fish $9.95 Time Remaining: 18h 18m

Tropical Fish Machine Embroidery Designs CD PES ocean clown fish $9.95 Time Remaining: 18h 18m

Tropical Fish Machine Embroidery Designs CD PES ocean clown fish $9.95 Time Remaining: 18h 18m

Tropical Fish Machine Embroidery Designs CD PES HUS JEF VIP DST $9.95 Time Remaining: 18h 18m

Fish Font Machine Embroidery Designs $3.49 Time Remaining: 1d 3h 37m Buy It Now for only: $3.49

Cute Aqua Fish Kids machine embroidery designs PES FSH5 $9.99 Time Remaining: 1d 10h 50m

New Funkey Fish kids Machine embroidery designs FSH2 $9.99 Time Remaining: 1d 17h

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 2d 1h 13m Buy It Now for only: $7.99

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 2d 1h 13m Buy It Now for only: $7.99

Tropical Fish Machine Embroidery Designs CD $9.95 Time Remaining: 2d 5h 40m

Tropical Fish Machine Embroidery Designs set $9.95 Time Remaining: 2d 6h 24m

Gold Fish machine embroidery designs set 4x4 hoop $5.99 Time Remaining: 2d 11h 6m

FISHING LURES 4X4 MACHINE EMBROIDERY DESIGNS ON CD $9.99 Time Remaining: 2d 11h 14m Buy It Now for only: $9.99

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 2d 15h 29m Buy It Now for only: $7.99

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 2d 15h 29m Buy It Now for only: $7.99

Tropical Fish Machine Embroidery Designs set PES HUS JEF VIP DST $9.95 Time Remaining: 2d 16h 22m

Ocean Fish Machine Embroidery Designs CD $9.95 Time Remaining: 2d 16h 24m

FANCY FISH V3 4X4 LD MACHINE EMBROIDERY DESIGNS $7.49 Time Remaining: 3d 3h 9m Buy It Now for only: $7.49

FANCY FISH V3 5X7 LD MACHINE EMBROIDERY DESIGNS $7.49 Time Remaining: 3d 3h 9m Buy It Now for only: $7.49

FISHING HIKING CAMPING LD MACHINE EMBROIDERY DESIGNS $7.49 Time Remaining: 3d 3h 9m Buy It Now for only: $7.49

Bears Fishing Machine Emb Redwork and Trapunto CD 10 designs in 3 sizes 30 total $18.00 Time Remaining: 3d 5h 21m Buy It Now for only: $18.00

Bernina Artista Embroidery Machine Card GAME FISH $58.00 Time Remaining: 3d 10h 38m Buy It Now for only: $58.00

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 4d 1h 6m Buy It Now for only: $7.99

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 4d 1h 6m Buy It Now for only: $7.99

Tropical Fish Machine Embroidery Designs CD PES HUS JEF VIP DST $9.95 Time Remaining: 4d 5h 36m

Tropical Fish Machine Embroidery Designs CD PES ocean clown fish $9.95 Time Remaining: 4d 6h 12m

Tropical Fish Machine Embroidery Designs CD PES ocean clown fish $9.95 Time Remaining: 4d 6h 16m

Tropical Fish Machine Embroidery Designs set $9.95 Time Remaining: 4d 6h 21m

Bears Fishing Machine Emb Redwork Trapunto Design CD 10 designs in 3 sizes $9.99 Time Remaining: 4d 9h 48m

DAKOTA COLLECTIBLES SPORTS GAME MACHINE EMBROIDERY CD HUNTING FISHING $39.99 Time Remaining: 4d 12h 31m Buy It Now for only: $49.99

Tropical Fish and More Machine Embroidery Designs $4.99 Time Remaining: 4d 15h 35m Buy It Now for only: $7.99

60+ Fish Machine Embroidery Designs $4.99 Time Remaining: 4d 15h 35m Buy It Now for only: $7.99

Tropical Fish Machine Embroidery Designs CD PES HUS JEF VIP DST $9.95 Time Remaining: 4d 16h 9m

Ocean Fish Machine Embroidery Designs set $9.95 Time Remaining: 4d 16h 12m

Tropical Fish Machine Embroidery Designs set $9.95 Time Remaining: 4d 17h 32m

Tropical Fish Machine Embroidery Designs CD $9.95 Time Remaining: 4d 17h 32m

FANCY FISH 13 MACHINE EMBROIDERY DESIGNS AzEB $19.95 Time Remaining: 5d 5h 9m Buy It Now for only: $19.95

DINOS FISH FROGS COLL MACHINE EMBROIDERY DESIGNS $29.99 Time Remaining: 5d 5h 51m Buy It Now for only: $29.99